Our config.txt is replaced (mv'ed to config.txt.old, maybe by alpine's
initramfs-generator?) and so our configuration does not have an
effect. Put our config stuff in usercfg.txt instead, which is included
by config.txt.

Also update vmlinuz name, should be vmlinuz-rpi and not vmlinuz-rpi4.

Prepare for better device categorization by moving everything to testing
subdir first.

[skip-ci]: chicken-egg problem: passing pmaports CI depends on pmbootstrap MR
				depends on this MR

Related: postmarketos#16

As mentioned in #151, the Raspberry Pi Zero doesn't have an ethernet
port, so this makes it quite complicated to connect to the device from
a fresh install, because wifi/bluetooth/serial/... are not configured.
With this new package, the device starts with the microUSB configured as
ethernet and the dhcpd is running on the RPI, so you can simply connect
it to a computer and connect with SSH in order to configure it.