Makefile: add endian link flag to u-boot-elf.o

fix compile error when using a little-endian to build with configs:
	sfr_nb4-ser_ram_defconfig
	comtrend_wap5813n_ram_defconfig
	comtrend_ar5387un_ram_defconfig
	comtrend_ct5361_ram_defconfig
	comtrend_vr3032u_ram_defconfig
	comtrend_ar5315u_ram_defconfig
	huawei_hg556a_ram_defconfig
	netgear_dgnd3700v2_ram_defconfig

Signed-off-by: Du Huanpeng <dhu@hodcarrier.org>