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>
diff --git a/include/configs/meson-gxbb-common.h b/include/configs/meson-gxbb-common.h
index cc2b5b6..997ce2d 100644
--- a/include/configs/meson-gxbb-common.h
+++ b/include/configs/meson-gxbb-common.h
@@ -39,6 +39,9 @@
 #include <config_distro_defaults.h>
 
 #define BOOT_TARGET_DEVICES(func) \
+	func(MMC, mmc, 0) \
+	func(MMC, mmc, 1) \
+	func(MMC, mmc, 2) \
 	func(DHCP, dhcp, na)
 
 #include <config_distro_bootcmd.h>