cortex-m: CONFIG_RO_HEAD_ROOM should only affect RO image
cortex-m: CONFIG_RO_HEAD_ROOM should only affect RO image

When we add head room to the RO image, it's generally to provide
a chip-specific boot header of some sort. That header is only
needed for the RO image, not the RW image. The macro name implies
this, but this CL makes it so.

BRANCH=none
BUG=chrome-os-partner:43025, chrome-os-partner:44625
TEST=the cr50 image validates *and jumps to* the RW image at boot.

Change-Id: I0e5b2c32e232418970e01c7409ddcbbabd4786d5
Signed-off-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/296451
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>

coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec