base-files: bring back nand_do_upgrade_success

base-files: bring back nand_do_upgrade_success

Several Broadcom targets were using the nand_do_upgrade_success
shell function which has been removed by commit e25e6d8e54
("base-files: fix and clean up nand sysupgrade code"). Refactor the
new nand_do_upgrade to bring back nand_do_upgrade_success with the
behavior expected by those users.

Fixes: e25e6d8e54 ("base-files: fix and clean up nand sysupgrade code")
Reported-by: Chen Minqiang <[email protected]>
Signed-off-by: Daniel Golle <[email protected]>