MIPS: zboot: Consolidate compiler flag filtering.

Al Viro noticed that we were using two different methods to filter out
flags from KBUILD_CFLAGS.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Reported-by: Al Viro <viro@zeniv.linux.org.uk>

arch/mips/boot/compressed/Makefile

@@ -18,7 +18,7 @@ include $(srctree)/arch/mips/Kbuild.platforms
 BOOT_HEAP_SIZE := 0x400000
 
 # Disable Function Tracer
-KBUILD_CFLAGS := $(shell echo $(KBUILD_CFLAGS) | sed -e "s/-pg//")
+KBUILD_CFLAGS := $(filter-out -pg, $(KBUILD_CFLAGS))
 
 KBUILD_CFLAGS := $(filter-out -fstack-protector, $(KBUILD_CFLAGS))