cbfstool: move iself() to eflheaders.c

The only user of iself() was in elfheaders.c. Move it there,
and make it local to the compilation unit.

Change-Id: I0d919ce372f6e2fce75885fb4fcba20d985979b3
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/5369
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Marc Jones <marc.jones@se-eng.com>

util/cbfstool/elfheaders.c

@@ -121,6 +121,13 @@
  * in the buffer struct.
  */
 
+
+static int iself(const void *input)
+{
+	const Elf32_Ehdr *ehdr = input;
+	return !memcmp(ehdr->e_ident, ELFMAG, 4);
+}
+
 /* Get the ident array, so we can figure out
  * endian-ness, word size, and in future other useful
  * parameters