Use hex (x) type argument for fdtput when writing bootdelay

This ensures the correct value (-2) is set on both 32-bit and 64-bit
systems

Signed-off-by: James Thomas <james.thomas@codethink.co.uk>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Add a --post-flash-cmd command line argument

This allows a user to specify the u-boot command to run after flashing
is complete (e.g 'ums 0 mmc 0'), defaults to 'reset' if not set

Signed-off-by: James Thomas <james.thomas@codethink.co.uk>
Signed-off-by: Stephen Warren <swarren@nvidia.com>