tplink-safeloader: add TP-Link Deco S4 v2 support

tplink-safeloader: add TP-Link Deco S4 v2 support

Support creating images for TP-Link Deco S4R v2.

Original partition layout from OEM image:
 partition fs-uboot base 0x00000 size 0x80000
 partition product-info base 0x80000 size 0x05000
 partition default-mac base 0x85000 size 0x01000
 partition device-id base 0x86000 size 0x01000
 partition support-list base 0x87000 size 0x10000
 partition user-config base 0xa7000 size 0x10000
 partition device-config base 0xb7000 size 0x10000
 partition group-info base 0xc7000 size 0x10000
 partition partition-table base 0xd7000 size 0x02000
 partition soft-version base 0xd9000 size 0x10000
 partition profile base 0xe9000 size 0x10000
 partition default-config base 0xf9000 size 0x10000
 partition url-sig base 0x1e0000 size 0x10000
 partition radio base 0x1f0000 size 0x10000
 partition os-image base 0x200000 size 0x200000
 partition file-system base 0x400000 size 0xc00000

The 'os-image' and 'file-system' partitions were merged into 'firmware'
to make use of the automatic mtd split.

Signed-off-by: Nick French <nickfrench@gmail.com>