tools: reorganize dependencies, fix build after deleting staging dir

A lot of packages (especially those using libtool) depend on sed being
available. xz fails on an incremental build pretty early if it's not
there.

Clean up handling if essential core tools (patch, tar, xz), build them
in parallel and make them depend on sed

Signed-off-by: Felix Fietkau <nbd@nbd.name>