serial: Fix Makefile during SPL and TPL build

This patch fixes a situation where CONFIG_DM_SERIAL is enabled for
regular U-Boot and SPL, but not for TPL. In that case, the build
will try to include serial-uclass into the TPL nonetheless, because
CONFIG_DM_SERIAL is set.

The solution is to check if the build is for SPL or TPL and in that
case, check if CONFIG_$(SPL_TPL_)DM_SERIAL is also set. Only in that
case, include serial-uclass.c . If the build is for regular U-Boot,
CONFIG_BUILD is not set, so only check if CONFIG_DM_SERIAL is set
and if so, include serial-uclass.c

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>