Switch everything to platforms

This patch switches the codebase over from using the "cpu"
mechanism to using bazel platforms. See
https://docs.bazel.build/versions/master/platforms.html for some more
information.

Most of the substantial changes are in //tools. Instead of using
`cc_toolchain_suite` rules, we now use regular `toolchain` rules that
are registered in the WORKSPACE. That also means that bazel now uses
the target platform to select the compiler.

All --cpu=* arguments should now be --config=* arguments. For example,
`--cpu=roborio` should now be `--config=roborio`. The CI script and
all documentation has been updated to reflect that.

The remainder of the changes revolve around tagging all targets with
`target_compatible_with`.  The old mechanism allowed us to specify
repo-wide defaults. The new mechanism does not. That means every
target that didn't have any compatibility specified, now requires
compatibility with `@platforms//os:linux`.

I used buildozer for the vast majority of `target_compatible_with`
changes. buildozer automatically buildifies any BUILD files it
touches. That means this patch also contains a few non-functional
changes that I was too lazy to remove.

Change-Id: I66d6e6ad9161520ee397597cdb492585820a3acd
diff --git a/motors/BUILD b/motors/BUILD
index 8fe5ff6..b606bf7 100644
--- a/motors/BUILD
+++ b/motors/BUILD
@@ -1,5 +1,4 @@
 load("//motors:macros.bzl", "hex_from_elf")
-load("//tools:environments.bzl", "mcu_cpus")
 
 cc_library(
     name = "motor",
@@ -9,7 +8,7 @@
     hdrs = [
         "motor.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         ":algorithms",
@@ -27,7 +26,7 @@
     hdrs = [
         "util.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         "//motors/core",
@@ -52,6 +51,7 @@
         "$@",
         "$<",
     ]),
+    target_compatible_with = ["@platforms//os:linux"],
     tools = [
         "@pandoc",
         "@pandoc//:all_files",
@@ -66,7 +66,6 @@
     hdrs = [
         "algorithms.h",
     ],
-    compatible_with = mcu_cpus,
 )
 
 cc_test(
@@ -74,6 +73,7 @@
     srcs = [
         "algorithms_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":algorithms",
         "//aos/testing:googletest",
@@ -88,7 +88,6 @@
     hdrs = [
         "math.h",
     ],
-    compatible_with = mcu_cpus,
     visibility = ["//visibility:public"],
 )
 
@@ -97,6 +96,7 @@
     srcs = [
         "math_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":math",
         "//aos/testing:googletest",
@@ -108,7 +108,7 @@
     srcs = [
         "button_board.cc",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     deps = [
         ":util",
         "//motors/core",
@@ -123,7 +123,7 @@
 
 hex_from_elf(
     name = "button_board",
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
 )
 
 cc_binary(
@@ -132,7 +132,7 @@
         "simple_receiver.cc",
     ],
     copts = ["-Wno-type-limits"],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     deps = [
         ":util",
         "//motors/core",
@@ -146,7 +146,7 @@
 
 hex_from_elf(
     name = "simpler_receiver",
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
 )
 
 cc_binary(
@@ -155,7 +155,7 @@
         "simpler_receiver.cc",
     ],
     copts = ["-Wno-type-limits"],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     deps = [
         ":util",
         "//motors/core",
@@ -167,11 +167,12 @@
 
 hex_from_elf(
     name = "simple_receiver",
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
 )
 
 py_library(
     name = "python_init",
     srcs = ["__init__.py"],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )