fastboot: add command to select the eMMC boot configuration

Add command oem bootbus which executes the command
``mmc bootbus <id> <arg>`` on the current fastboot mmc device
(<i> = CONFIG_FASTBOOT_FLASH_MMC_DEV) to set the eMMC boot
configuration on first update, with
<arg> =  boot_bus_width reset_boot_bus_width boot_mode

$> fastboot oem bootbus:<boot_bus_width> <reset_boot_bus_width> <boot_mode>

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>