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/modules.gni b/modules.gni
index fbbee69..6a65579 100644
--- a/modules.gni
+++ b/modules.gni
@@ -18,6 +18,7 @@
 
 dir_pw_base64 = "$dir_pigweed/pw_base64"
 dir_pw_bloat = "$dir_pigweed/pw_bloat"
+dir_pw_boot_armv7m = "$dir_pigweed/pw_boot_armv7m"
 dir_pw_build = "$dir_pigweed/pw_build"
 dir_pw_cpu_exception = "$dir_pigweed/pw_cpu_exception"
 dir_pw_cpu_exception_armv7m = "$dir_pigweed/pw_cpu_exception_armv7m"