MA-14318-2 Support dual bootloader for xen

Add new targets which only include u-boot proper to
support dual bootloader for xen.

Test: build and boot on imx8qm_mek.
Signed-off-by: Ji Luo <ji.luo@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>

iMX8QM/scripts/android.mak

+flash_b0_xen_uboot: u-boot-hash.bin
+	./$(MKIMG) -soc QM -rev B0 -c -ap u-boot-hash.bin a53 0x81080000 -out u-boot-xen-container.img \
+
 flash_b0_spl_container_m4_1_trusty: $(MKIMG) mx8qm-ahab-container.img scfw_tcm.bin u-boot-atf-container.img m4_1_image.bin tee.bin u-boot-spl.bin
 	./$(MKIMG) -soc QM -rev B0 -append mx8qm-ahab-container.img -c -flags 0x01200000 -scfw scfw_tcm.bin -p4 -m4 m4_1_image.bin 1 0x88800000 -ap u-boot-spl.bin a53 0x00100000 -out flash.bin
 	cp flash.bin boot-spl-container.img