updated doc references of --uboot to --dtb

The --uboot option has been replaced by the --dtb
option (See mk_mmc.sh line 1348 for more info).
All documentation related references to --uboot have
been replaced with --dtb.

The code for generating an error if --uboot is
used in mk_mmc.sh has been left in place.
Merge pull request #29 from jmahler/master

updated doc references of --uboot to --dtb