meson: gxbb: enable MMC as boot target

meson: gxbb: enable MMC as boot target

To enable automatic booting from SD card or eMMC the MMC
devices 0, 1, and 2 are added to the BOOT_TARGET_DEVICES.

Booting from SD card, eMMC, and DHCP are tried in sequence.
A missing or failing device is gracefully handled.

Cc: Andreas Färber <afaerber@suse.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Vagrant Cascadian <vagrant@debian.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Tested-by: Andreas Färber <afaerber@suse.de>