build: Pull -DBUILD_TAG into separate ifdef

Currently the base setting for CFLAGS is split in two possibilities,
one with -DBUILD_TAG appended at the end and one without, the rest of
CFLAGS is the same in both cases. Change this so CFLAGS are always set
and the -DBUILD_TAG is appended in separate ifdef.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Tom Rini <trini@ti.com>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>