Add a way to pass in define flag when 'make'
Add a way to pass in define flag when 'make'

In order to pass in different config from ebuild and enable easier
debugging/testing, let's add a way to pass in define flags to make.

BUG=chrome-os-partner:9936
TEST='V=1 EXTRA_CFLAGS=-DFACTORY_MODE make' and see '-DFACTORY_MODE'.

Change-Id: I18dc5a3dc61bda1e63e96081def3f077ba431938

fourier49/BIZ_EC,akappy7/ChromeOS_EC_LED_Diagnostics,alterapraxisptyltd/chromium-ec,alterapraxisptyltd/chromium-ec,eatbyte/chromium-ec,gelraen/cros-ec,thehobn/ec,md5555/ec,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,thehobn/ec,gelraen/cros-ec,mtk09422/chromiumos-platform-ec,coreboot/chrome-ec,coreboot/chrome-ec,thehobn/ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,eatbyte/chromium-ec,longsleep/ec,longsleep/ec,gelraen/cros-ec,mtk09422/chromiumos-platform-ec,eatbyte/chromium-ec,akappy7/ChromeOS_EC_LED_Diagnostics,eatbyte/chromium-ec,alterapraxisptyltd/chromium-ec,longsleep/ec,fourier49/BZ_DEV_EC,longsleep/ec,coreboot/chrome-ec,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BIZ_EC,alterapraxisptyltd/chromium-ec,md5555/ec,fourier49/BZ_DEV_EC,coreboot/chrome-ec,coreboot/chrome-ec,mtk09422/chromiumos-platform-ec,md5555/ec,coreboot/chrome-ec,fourier49/BIZ_EC,mtk09422/chromiumos-platform-ec,gelraen/cros-ec,thehobn/ec