ramips: enable fw_printenv for D-Link COVR-X1860 A1

ramips: enable fw_printenv for D-Link COVR-X1860 A1

Vendor U-Boot stores its environment at offset 0x2000 inside the
"config" partition (size 0x4000). The offset is not erase-block
aligned, so the fw_env tool's alignment check rejects the natural
secsize=0x20000. Declaring secsize=0x2000 passes the check, lets
fw_printenv read the real env (factory_mac, hw_version,
bundle_number, ...), and causes fw_setenv to fail cleanly on erase
rather than corrupting anything (the kernel rejects sub-erase-block
MEMERASE).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Florian Maurer <fmaurer@disroot.org>
Link: https://github.com/openwrt/openwrt/pull/23192
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>