crosfw: Create eMMC magic flasher
crosfw: Create eMMC magic flasher

The '-M' option allows building a magic flasher to flash the eMMC boot
partition.

BUG=chromium:254233
TEST='crosfw -b daisy_spring -VSF' flashes to SPI flash
'crosfw -b daisy_spring -VSM' flashes to eMMC

Change-Id: Idff1ff6ab627870c13e039cc26680468ed14f416
Signed-off-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/63998
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite