MIPS: add support for generating u-boot.elf

Define PLATFORM_ELFFLAGS for MIPS in order to be able to generate u-boot.elf
Signed-off-by: NÁlvaro Fernández Rojas <noltari@gmail.com>

arch/mips/config.mk

@@ -36,6 +36,8 @@ OBJCOPYFLAGS		+= -O $(64bit-bfd)
 endif
 
 PLATFORM_CPPFLAGS += -D__MIPS__
+PLATFORM_ELFENTRY = "__start"
+PLATFORM_ELFFLAGS += -B mips $(OBJCOPYFLAGS)
 
 #
 # From Linux arch/mips/Makefile