cbfstool: add ELF header initialization helper

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>