ath79: mikrotik: erase firmware on SPI NOR before install

ath79: mikrotik: erase firmware on SPI NOR before install

On Mikrotik SPI NOR devices, the firmware partition must be erased when
flashing from stock firmware, otherwise leftover bits (in particular a
kernel signature) can trigger a boot loop.

When booted from initramfs (the only way to install OpenWRT on these
devices), this patch unconditionally erases the firmware partition in
the do_upgrade() stage for all supported SPI NOR devices.

This is forward-ported from ed49d0876 and 20452a8db

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>