bcm47xx: base-files: hack sysinfo to allow ASU sysupgrades

bcm47xx: base-files: hack sysinfo to allow ASU sysupgrades

This old platform(no DTS) is using nvram numeric fields (or "unknown"
string) for its board_name [1]. Allow ASU sysupgrades by preserving
that value in /tmp/sysinfo/boardtype for board-detection/configuration
logic, and derive a canonical board_name from /tmp/sysinfo/model in
the form "vendor,device-variant".

Only a few target profile image names are not consistent in the last
suffix, uniform them.

A few images with (NA) and (ROW) variants are only meant to be
compatible with the upgrade process in the OEM firmware using these
NETGEAR_BOARD_ID and NETGEAR_REGION fields but the images are
compatibles(only if the NETGEAR_BOARD_ID is shared,due to platform
check) [2] (i.e. netgear_wnr3500l).
Add SUPPORTED_DEVICES to one of these variant in order to allow ASU
sysupgrade profile identification.

*Since this target has never implemented fwtool's SUPPORTED_DEVICES
metadata check, there is no risk of breaking forceless sysupgrade with
new board_name values.

[1]: bcm47xx board info https://github.com/gregkh/linux/blob/master/arch/mips/bcm47xx/board.c
[2]: sysupgrade platform check https://github.com/openwrt/openwrt/blob/main/target/linux/bcm47xx/base-files/lib/upgrade/platform.sh

Fixes: 7d10f2c1e851 "brcm47xx: rework model detection"
Fixes: https://github.com/openwrt/asu/issues/419 (Netgear wnr3500L)
Closes: https://github.com/openwrt/asu/issues/878
Fixes: https://github.com/openwrt/asu/issues/1042 (Asus RT-N16)
Fixes: https://forum.openwrt.org/t/luci-attended-sysupgrade-support-thread/230552/115 (Asus RT-N16)
Signed-off-by: Mario Andrés Pérez <mapb_@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/21147
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>