metaknight_legacy: Enable brief debug asserts
metaknight_legacy: Enable brief debug asserts

Metaknight_legacy is running out of space.  This commit simply enables
CONFIG_DEBUG_ASSERT_BRIEF in order to free up some flash space.

BUG=None
BRANCH=dedede
TEST=`make -j buildall`

Signed-off-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@google.com>
Change-Id: Icfb16548913a271d571c993d1fefbe1cea9c3403
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2582593
Tested-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>
Reviewed-by: Diana Z <cb2016afc7183e5356c690dc63b4ba817470501a@chromium.org>
Commit-Queue: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>

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