target: Disable building devices with 8M, 16M and 32M RAM

[Felix Baumann]

Hello,

I suggest disabling the build of devices with too little amount of ram
before the new release.
People who still need current openwrt versions for these devices
probably have to build them themselves in order for them to run with
this little amount of ram. (or stay on older releases instead)
Feedback is welcome.

This change hopefully prevents issues like this one from being reported:
https://github.com/openwrt/openwrt/issues/12554
Let's save people from themselves and free (some) buildbot ressources at
the same time.

I used the ToH as my primary source but I also looked up lots of devices
for some of the older targets, when the devices weren't present in the ToH.
I most certaily missed a few 32M devices (due to missing ToH entries)
but these should be most of them.
I was surprised to find 8M and 16M RAM devices that are still being
built on a daily basis.

I couldn't find any ram specs on the following devices:
D-Link DWL-3150: amount of ram unknown, likely only 16M like the others
(last remaining device in subtarget bcm47xx/legacy)
Argus ATP-52B, Omnima MiniEMBWiFi, Hauppauge Broadway, NexAira Business
Class II : amount of ram unknown (subtarget ramips/rt3050x)

PR on GitHub:
https://github.com/openwrt/openwrt/pull/12672

Patch file:
https://github.com/openwrt/openwrt/pull/12672.patch
I know you prefer patches to be embedded into the mail on this list.
Maybe for another patch.

--
Regards
Felix Baumann