Fix LM4 flashing in flash_ec
Fix LM4 flashing in flash_ec

This fixes the bug that SRC_ROOT is not set. This should be the last fix
for flash_ec. DEFAULT_BOARD is intentionally left as is. For developers
who don't want to use --board option every time, they need to set
DEFAULT_BOARD in their environment variables.

BUG=chromium:397202
TEST=util/flash_ec --board=link
BRANCH=None

Change-Id: If23f73adbd37f2a79cb5176e3665562e278f46db
Signed-off-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/210523
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Reviewed-by: Gwendal Grignou <190c5a5edfcf62e0246559bf31fa7548674de3ad@chromium.org>

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