kbuild: add real-prereqs shorthand for $(filter-out FORCE, $^)

Backport from kernel
commit afa974b77128 ("kbuild: add real-prereqs shorthand for $(filter-out FORCE,$^)")

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 6d0cb8e..4861b13 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -546,7 +546,7 @@
 # append the size as a 32-bit littleendian number as gzip does.
 size_append = printf $(shell						\
 dec_size=0;								\
-for F in $(filter-out FORCE,$^); do
+for F in $(real-prereqs); do
 	fsize=$$(stat -c "%s" $$F);					\
 	dec_size=$$(expr $$dec_size + $$fsize);				\
 done;									\
@@ -560,19 +560,19 @@
 )
 
 quiet_cmd_bzip2 = BZIP2   $@
-cmd_bzip2 = (cat $(filter-out FORCE,$^) | bzip2 -9 && $(size_append)) > $@
+cmd_bzip2 = (cat $(real-prereqs) | bzip2 -9 && $(size_append)) > $@
 
 # Lzma
 # ---------------------------------------------------------------------------
 
 quiet_cmd_lzma = LZMA    $@
-cmd_lzma = (cat $(filter-out FORCE,$^) | lzma -9 && $(size_append)) > $@
+cmd_lzma = (cat $(real-prereqs) | lzma -9 && $(size_append)) > $@
 
 quiet_cmd_lzo = LZO     $@
-cmd_lzo = (cat $(filter-out FORCE,$^) | lzop -9 && $(size_append)) > $@
+cmd_lzo = (cat $(real-prereqs) | lzop -9 && $(size_append)) > $@
 
 quiet_cmd_lz4 = LZ4     $@
-cmd_lz4 = (cat $(filter-out FORCE,$^) | lz4c -l -c1 stdin stdout && \
+cmd_lz4 = (cat $(real-prereqs) | lz4c -l -c1 stdin stdout && \
 	$(size_append)) > $@
 
 # U-Boot mkimage