bcm53xx: sysupgrade: refactor handling different firmware formats

bcm53xx: sysupgrade: refactor handling different firmware formats

List of supported formats grew over time and implementation got a bit
messy. There are multiple functions with format-specific parameters and
commands.

Refactor it by making platform_identify() setup all required info right
after detecting firmware format. This simplifies formats handling in
platform_other_check_image() and platform_do_upgrade() a lot.

This has been tested on:
1. SmartRG SR400ac (TRX): non-NAND sysupgrade
2. Netgear R8000 (CHK): NAND aware and standard sysupgrade-s
3. D-Link DIR-885L (Seama): NAND aware and standard sysupgrade-s
4. Luxul XWR-3150 (LXL): NAND aware and standard sysupgrade-s

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>