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.
Fixes: https://gitlab.com/postmarketOS/pmaports/-/issues/2471
[ci:skip-build]: already built successfully in CI
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
Henrik Grimler