uboot-imx

ARM: socfpga: remove redundant config.mk

Because all the SOCFPGA boards define CONFIG_SPL_FRAMEWORK
(see include/configs/socfpga_common.h), u-boot.img is automatically
added to the target image list by the top Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>