mainboard/google/coral: Overwrite family code for coral models.

This patch assigns the code of coral family, such that,
the 'mosys platform family' returns 'Google_Coral'.

BUG=b:64467244, b:64501879
BRANCH=none
TEST=Examine 'mosys platform family' w/ new firmware.

Change-Id: I1d8f8ca2166a1d80855608cf5b64b0cc7bf3dc93
Signed-off-by: Harry Pan <harry.pan@intel.com>
Reviewed-on: https://review.coreboot.org/21136
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/mainboard/google/reef/Kconfig

@@ -77,6 +77,7 @@ config MAINBOARD_PART_NUMBER
 
 config MAINBOARD_FAMILY
 	string
+	default "Google_Coral" if BOARD_GOOGLE_CORAL
 	default "Google_Reef"
 
 config GBB_HWID