ramips: D-Link Dxx-1xx0 image fixes

ramips: D-Link Dxx-1xx0 image fixes

Three fixes for D-Link DAP-1620 rev B and its twin D-Link DRA-1360:

1. `uboot-envtools` is removed from default package list.
2. Makefile variable is doubly escaped, i.e. `$$$$(DLINK_HWID)`.
3. Previously the size of `factory.bin` was always 10.5 MiB, same as
D-Link firmwares. This commit makes it possible to use smaller images
(with no lost space due to padding) as well as larger images. Tested
successfully flashing a 6.5 MiB image and a 14.5 MiB image.

Recall that factory images need to be installed via D-Link Web Recovery
(at http://192.168.0.50/, server ignores pings and DHCP requests).

P.S.
I implemented the OEM firmware encryption algorithm, so firmware can be
flashed via OEM firmware, but after successful flashing the device
reboots to web recovery, so further debugging is required.

Signed-off-by: Rani Hod <rani.hod@gmail.com>