Support using 'webtop' as the system partition

[timangus]

The Motorola Droid 4's system partition is ~600Mb, which is too small
to be able to contain modern ROMs based off of Android 7 and a Gapps
package. This change adds an extra 'slot' to the recovery that allows
the repurposing of the otherwise unused 'webtop' partition to be used
in place of the stock system parition.

[Dead2]

I wish there was also a slot that let us use webtop as system, cache as cache, but lets us make userdata use a loop image. Sort of a hybrid approach that does not actually conflict with the stock slot.

[timangus]

You could do that I guess, but would you not then suffer the performance problems that you get with loopback partitions? Perhaps to a lesser extent I suppose, but if you're going to make just one partition loopback, userdata is probably the worst one to pick.

[Dead2]

Sure, but there are no other 3GB+ partitions that do not conflict. It also saves space on the internal "sdcard" :)
Ideally this would all be completely configurable in the gui, but that is of course a lot more work.

[Dead2]

(I'd have added an issue, but that has been disabled)
An unrelated thing I'd like to see added to the rescue image is fsck_vfat, since the internal sdcard can get a corrupt filesystem, but you cannot repair it from a fully booted android system. One of my safeboot images got corrupted, so while ls showed 3GB size, du would show 16 Exabytes. Some other files were also not usable. Formatting the filesystem is of course possible, but sometimes you want to rescue some of the data first :)