cbfstool: add ELF header initialization helper

In order for one to extract ELF files from cbfs it's
helpful to have common code which creates a default
executable ELF header for the provided constraints.

BUG=None
TEST=With follow up patch am able to extract out romstage
     as an ELF file.

Change-Id: Ib8f2456f41b79c6c0430861e33e8b909725013f1
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/12218
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/cbfstool/elfparsing.h

@@ -76,6 +76,17 @@ elf_headers(const struct buffer *pinput,
 /* ELF writing support. */
 struct elf_writer;
 
+/*
+ * Initialize a 64-bit ELF header provided the inputs. While the structure
+ * is a 64-bit header one can specify a 32-bit machine. The 64-bit version
+ * is just used as a common structure. If one wants to specify the entry
+ * point, for example, the caller can set it after filling in the common
+ * bits. The machine, nbits, and endian values should be from the ELF
+ * definitions (e.g. EM_386, ELFCLASS32, and ELFDATA2LSB) found in elf.h
+ * with no endian conversion required.
+ */
+void elf_init_eheader(Elf64_Ehdr *ehdr, int machine, int nbits, int endian);
+
 /*
  * Initialize a new ELF writer. Deafult machine type, endianness, etc is
  * copied from the passed in Elf64_Ehdr. Returns NULL on failure, valid