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: Álvaro Fernández Rojas <noltari@gmail.com>
diff --git a/arch/mips/config.mk b/arch/mips/config.mk
index dcd3460..2c72c15 100644
--- a/arch/mips/config.mk
+++ b/arch/mips/config.mk
@@ -36,6 +36,8 @@
 endif
 
 PLATFORM_CPPFLAGS += -D__MIPS__
+PLATFORM_ELFENTRY = "__start"
+PLATFORM_ELFFLAGS += -B mips $(OBJCOPYFLAGS)
 
 #
 # From Linux arch/mips/Makefile