makefile: fix flash image size reporting
makefile: fix flash image size reporting

The Makefile uses awk to find a section named FLASH in the output map
file and parses the size of the section to determine the free space
remaining.

NPCX RO images include sections named FLASH and FLASH_HDR. Modify the
awk command so the FLASH_HDR section isn't used by mistake.

BUG=none
BRANCH=none
TEST=make buildall

Signed-off-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>
Change-Id: I3e420fbb45fdebc4f1e2578f819bbbce4994d2a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2640873
Commit-Queue: caveh jalali <08e24a66aefda7a55ca7d73f8903e0fb16eec763@chromium.org>
Reviewed-by: Abe Levkoy <2e1ad7039298f93262cf06bbb457275fb3e2ecb5@chromium.org>
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Reviewed-by: caveh jalali <08e24a66aefda7a55ca7d73f8903e0fb16eec763@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec