prepare-boot-media.sh: merge fixes from mksdboot_omap5.sh

This script is based on mksdboot_omap5.sh script (in meta-mentor-glsdk
layer) which has been updated to incorporate file-names from MEL installer
and to work with built-in card-readers. Those fixes should be applied
to this script as well.

Pull requests for mksdboot_omap5.sh:
  https://github.com/MentorEmbedded/meta-mentor-glsdk/pull/83
  https://github.com/MentorEmbedded/meta-mentor-glsdk/pull/84
  https://github.com/MentorEmbedded/meta-mentor-glsdk/pull/85
  https://github.com/MentorEmbedded/meta-mentor-glsdk/pull/86

Signed-off-by: Fahad Usman <fahad_usman@mentor.com>