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/pw_vars_default.gni b/pw_vars_default.gni
index c5458da..071aec1 100644
--- a/pw_vars_default.gni
+++ b/pw_vars_default.gni
@@ -92,10 +92,21 @@
 # empty (but defined) variable.
 #
 # All of these should default to empty strings. For target-specific defaults,
-# modify these variables in a target confiruation file.
+# modify these variables in a target configuration file.
+
+# Backend for the pw_boot module.
+dir_pw_boot_backend = ""
+
+# Backend for the pw_cpu_exception module.
+dir_pw_cpu_exception_backend = ""
 
 # Backend for the pw_dumb_io module.
 dir_pw_dumb_io_backend = ""
 
-# Backend for the pw_cpu_exception module.
-dir_pw_cpu_exception_backend = ""
+############################## MODULE CONFIGS ##################################
+
+# Module configuration options for pw_boot_armv7m.
+pw_boot_armv7m_config = {
+  # C Preprocessor defines used for linker script configuration.
+  defines = []
+}