write_firmware: Add support for emmc booting
write_firmware: Add support for emmc booting

This change adds support to write_firmware to generate the flash script
for Sdmmc booting.

BUG=None
TEST=Was able to flash an image to Sdmmc and boot it.

Change-Id: I7564b186b2c89bfa1162060f58b3b4ae7d8d9c0f
Signed-off-by: Rhyland Klein <9c6f026f68ffecc44f215af186ef513cd72b6ce4@nvidia.com>
Reviewed-on: https://gerrit.chromium.org/gerrit/17641
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

mgireesh05/dev-util,mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,marineam/coreos-dev-util