[ARM] Update ARM makefiles

[ARM] Update ARM makefiles

- Use -march=armv6 where available.
- CFLAGS_BOOT isn't needed - the decompressor uses CFLAGS, remove it.
- PROCESSOR is never used, remove it.
- LDFLAGS_BLOB is never used, remove it.
- Remove MACHINE macro.
- Correct comments for include/asm-arm/.arch target.
- Copy format of include/asm symlink rule.
- Arrange for configuration to be split before making
  include/asm-arm/arch symlink.
- This means that asm-offsets.s no longer depends on
  include/config/MARKER.