Set proper value for BOOTCFG
Set proper value for BOOTCFG

We choose PE2/USB1_CTL1 for BOOTCFG. If we somehow brick the EC, we can
pull this signal to ground and boot EC into boot loader to recover it.

BUG=chrome-os-partner:8769
TEST=On link:
      - Factory reset EC to clear BOOTCFG
      - Flash new EC binary and reboot
      - rw 0x400fe1d0 to check BOOTCFG value is correctly written
      - Boot normally and check EC is operating correctly
      - Pull the signal to ground and reset EC, check EC stays in boot
        loader. Check in this case we can program EC.
      - Attempt to write different value to BOOTCFG and check BOOTCFG
	doesn't change.

Change-Id: Ia6705114d495b18bd7ee4afc1e61e84a21b51198
Reviewed-on: https://gerrit.chromium.org/gerrit/27000
Reviewed-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Tested-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Commit-Ready: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>

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