dfu: add 'SCRIPT' entity

Define a new 'SCRIPT' type for DFU entities. The downloaded data are
treated as simple u-boot's scripts and executed with run_command_list()
function.

Flashing the 'SCRIPT' entity might result in changing the 'dfu_alt_info'
environment variable from the flashed script, so add a global variable
for tracking the potential need to reinitialize the dfu_alt_info related
structures.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>