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>