usb/gadget: fastboot: add support for flash command

- implement 'fastboot flash' for eMMC devices

Signed-off-by: Steve Rae <srae@broadcom.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/doc/README.android-fastboot b/doc/README.android-fastboot
index 4045727..1677609 100644
--- a/doc/README.android-fastboot
+++ b/doc/README.android-fastboot
@@ -6,8 +6,9 @@
 The protocol that is used over USB is described in
 README.android-fastboot-protocol in same directory.
 
-The current implementation does not yet support the flash and erase
-commands.
+The current implementation does not yet support the erase command or the
+"oem format" command, and there is minimal support for the flash command;
+it only supports eMMC devices.
 
 Client installation
 ===================