microblaze: add endianness options to LDFLAGS instead of LD

With the recent syntax extension, Kconfig is now able to evaluate the
compiler / toolchain capability.

However, accumulating flags to 'LD' is not compatible with the way
it works; 'LD' must be passed to Kconfig to call $(ld-option,...)
from Kconfig files.  If you tweak 'LD' in arch Makefile depending on
CONFIG_CPU_BIG_ENDIAN, this would end up with circular dependency
between Makefile and Kconfig.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/Makefile

@@ -40,11 +40,11 @@ CPUFLAGS-$(CONFIG_XILINX_MICROBLAZE0_USE_PCMP_INSTR) += -mxl-pattern-compare
 ifdef CONFIG_CPU_BIG_ENDIAN
 KBUILD_CFLAGS += -mbig-endian
 KBUILD_AFLAGS += -mbig-endian
-LD += -EB
+LDFLAGS += -EB
 else
 KBUILD_CFLAGS += -mlittle-endian
 KBUILD_AFLAGS += -mlittle-endian
-LD += -EL
+LDFLAGS += -EL
 endif
 
 CPUFLAGS-1 += $(call cc-option,-mcpu=v$(CPU_VER))