prepare-boot-media.sh: Initial commit

Take the mksdboot_dra7xx.sh script from meta-mentor-glsdk and adapt it to
prepare a bootable USB drive or SD card for Freescale p1/p2 series targets
like p1010rdb and p2020rdb.

Signed-off-by: Fahad Usman <fahad_usman@mentor.com>
Merge pull request #390 from fahadusman/make-boot-device-script

prepare-boot-media.sh: Initial commit