Update pico-sdk and bazelify it

This borrows heavily from work that Austin did to initially get this
stuff working.

Merge commit 'a1fdcd85e25a8a2b1e3ca5bc31e45bedf9e58266' into HEAD

Signed-off-by: Ravago Jones <ravagojones@gmail.com>
Change-Id: Id47d383bd5fb46664083b365890fd7bad5786c1f
diff --git a/third_party/pico-sdk/tools/elf2uf2/BUILD b/third_party/pico-sdk/tools/elf2uf2/BUILD
index 3c8cf78..1a985c8 100644
--- a/third_party/pico-sdk/tools/elf2uf2/BUILD
+++ b/third_party/pico-sdk/tools/elf2uf2/BUILD
@@ -8,6 +8,7 @@
         "-Wno-format-nonliteral",
         "-Wno-reorder",
         "-Wno-unused-parameter",
+        "-Wno-unused-variable",
         "-Wno-unused-function",
         "-Wno-type-limits",
     ],