tegra: bootcmd enhancements

tegra: bootcmd enhancements

Place the list of searched boot devices, file-system types, boot file
locations/prefixes, and boot script names into variables. This allows
the user to override them directly (e.g. to change boot order, or select
the specific values they use) without having to edit the main bootcmd
and script_boot variables.

The default boot order is changed from USB, MMC, DHCP to MMC, USB, DHCP.
This speeds up the typical MMC boot case. People who want USB boot can
now edit variable boot_targets appropriately.

Also, reformat BOOTCMDS_COMMON to fit within 80 columns.

Potential future enhancements might be:
* Allow boards to specify which of bootcmd_mmc0/... should be defined,
  based on the HW they contain.
* Allow boards to influence the order that the boot targets are added
  into the boot_targets variable.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>