pw_boot_armv7m: Initial commit

pw_boot_armv7m provides a generic linker script and startup script to
speed up device bringup on ARMv7-M based cores.

Change-Id: Ia4abe78ab1d6bde017608eeb46b2e780bf53dad1
diff --git a/docs/BUILD.gn b/docs/BUILD.gn
index dee8a15..c5325bb 100644
--- a/docs/BUILD.gn
+++ b/docs/BUILD.gn
@@ -41,6 +41,7 @@
     ":core_docs",
     ":target_docs",
     "$dir_pw_bloat:docs",
+    "$dir_pw_boot_armv7m:docs",
     "$dir_pw_build:docs",
     "$dir_pw_cpu_exception:docs",
     "$dir_pw_cpu_exception_armv7m:docs",