Make it possible to find the kernel config with "pmbootstrap kconfig
edit". While at it, also remove redundant -C "$builddir" arguments for
make (since prepare is running in $builddir already).
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