Add decap.sh: a simple shell script to remove headers of UEFI Capsule files

Change-Id: Ic479cf0d02be7ad562ee3a489213a6b2ef88e2c9
Signed-off-by: Stefan Tauner <stefan.tauner@gmx.at>
Reviewed-on: http://review.coreboot.org/6480
Tested-by: build bot (Jenkins)