pw_boot_cortex_m: Rename pw_boot_armv7m to pw_boot_cortex_m:armv7m

Since the implementation is closer to a generic Cortex-M implementation,
rename pw_boot_armv7m to pw_boot_cortex_m, and add a core-specific
target named "armv7m".

This matches pw_cpu_exception_cortex_m.

Toolchain definitions will need ot be changed from:
  pw_boot_BACKEND = dir_pw_boot_armv7m
to
  pw_boot_BACKEND = "$dir_pw_boot_cortex_m:arm7vm"

Change-Id: Id6f4bbf8e6c1ec89afa63de7c93be1f3afa59f09
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/54242
Commit-Queue: Scott James Remnant <keybuk@google.com>
Pigweed-Auto-Submit: Scott James Remnant <keybuk@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_sys_io_baremetal_lm3s6965evb/BUILD.gn b/pw_sys_io_baremetal_lm3s6965evb/BUILD.gn
index 5ff5e86..1fa0ecf 100644
--- a/pw_sys_io_baremetal_lm3s6965evb/BUILD.gn
+++ b/pw_sys_io_baremetal_lm3s6965evb/BUILD.gn
@@ -25,7 +25,7 @@
   public_configs = [ ":default_config" ]
   public = [ "public/pw_sys_io_baremetal_lm3s6965evb/init.h" ]
   public_deps = [
-    "$dir_pw_boot_armv7m",
+    "$dir_pw_boot_cortex_m:armv7m",
     "$dir_pw_preprocessor",
   ]
   deps = [