Draft: Revert "mkinitcpio: fix behavior generatedir"

This reverts commit 01c71e23.

This commit changed the semantics of initialize_buildroot and made some invocations of the function leave garbage in /tmp/mkinitcpio.XXXXXX, especially during tests. Restore the previous behaviour, and instead copy the built tree to _opttargetdir if the flag is passed. This should keep things working for mkinitcpio-generate-shutdown-ramfs.service.

Alternate working directories are always passed with -t, which exports a different TMPDIR, so there should be no need for the second argument on initialize_buildroot.

cc: @gekmihesg