mach-imx: bootaux: fix building with disabled bootelf

mach-imx: bootaux: fix building with disabled bootelf

If CMD_ELF disabled and IMX_BOOTAUX enabled, the u-boot building ends
up with a linking error [1]. Select LIB_ELF to fix the building
issue.

[1]
ld: /tmp/ccaF1rpv.ltrans0.ltrans.o: in function `do_bootaux':
arch/arm/mach-imx/imx_bootaux.c:108: undefined reference to `valid_elf_image'

Fixes: c0f037f6a2a ("mach-imx: bootaux: elf firmware support")
Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>