mkuboot: build two versions, one for rescue from eMMC

reform2-imx8mq/mkuboot.sh

@@ -14,7 +14,12 @@ cp mntreform-config .config
 
 export CROSS_COMPILE=aarch64-linux-gnu-
 export ARCH=arm
+
+# build rescue u-boot first (loads kernel from eMMC)
+make -j$(nproc) flash.bin KCPPFLAGS='-DMNTREFORM_BOOT_EMMC'
+cp flash.bin flash-rescue.bin
+
+# build normal u-boot second (loads kernel from SD card)
 make -j$(nproc) flash.bin
 
 cd ..
-