Makefile: Use common code for SPI_FLASH deprecation warning

Update the CONFIG_DM_SPI_FLASH check to use the 'deprecated' function.

Tested with vinco

Old message:

===================== WARNING ======================
This board does not use CONFIG_DM_SPI_FLASH. Please update
the board to use CONFIG_SPI_FLASH before the v2019.07 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
====================================================

New message:

===================== WARNING ======================
This board does not use CONFIG_DM_SPI_FLASH (Driver Model
for SPI flash). Please update the board to use
CONFIG_DM_SPI_FLASH before the v2019.07 release. Failure to
update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
====================================================

Signed-off-by: Simon Glass <sjg@chromium.org>