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 <ptpt52@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>