Set build_pkgs_on_install=True in pmbootstrap.cfg, if
POSTMARKETOS_ALLOW_LOCAL_PKGS is set. Otherwise, pmbootstrap will stop
if a local package needs to be built during the installation, with:

(010277) [11:42:32] (rootfs_raspberry-pi4) install postmarketos-base device-raspberry-pi4 breeze-icons mycroft-core plasma-mycroft postmarketos-ui-plasma-bigscreen
(010277) [11:42:32] ERROR: postmarketos-ui-plasma-bigscreen: could not find binary package, although it should exist for sure at this point in the code. Probably an APKBUILD subpackage parsing bug. Related: build.postmarketos.org#61

The pmbootstrap message isn't exactly helpful, so I'll make a
pmbootstrap patch to improve that as well.

The package is called "osimage", not "osimage-pmos-systemimage".

The '## build factory apks (1/3): osimage-pmos-systemimage' is invalid
now, as "build factory apks" is done in just two steps and the first
step already has its own log message. Also that package was renamed.

Add a script to create the three-layered factory image for the
PinePhone, based on Martijn's factorytest. factorytest was also used for
the Brave Heart and to flash the ubports CE. Add factorytest and related
pmaports, as well as dependencies that are only in Alpine testing (so we
can build the layer 3 image with v20.05 instead of edge). There is a lot
that could be optimized here, but that doesn't seem like a good use of
time.

Move some code from build.sh to common.sh, so it can be used by the new
script.

Do not compress the image, if it is set. This will be used by the
pinephone factory image script.

This information is stored in _pmb_recommends in the postmarketos-ui-*
APKBUILDs now, and used by pmbootstrap during install.

pmbootstrap zap -p is already being called in "build_image()".

Use a separate config file, pmbootstrap-images.cfg. Add a wrapper
function "pmbootstrap()", which calls pmbootstrap with the custom config
path, and another wrapper "pmbootstrap_default_cfg()" to run it with the
default config.

Adjust to rename in the merge request.

Instead of setting the hostname to "none", set it to the device name.

Use together with pmbootstrap!1951, or set
POSTMARKETOS_ALLOW_LOCAL_PKGS=1.

Closes: postmarketos#28