[Buildroot] Parallel top-level make and multiple rootfs types

[Jan Kundrát]

Hi,
I've enabled that unsupported feature for parallel top-level make by 
removing ".NOTPARALLEL". I'm doing this because it cuts my build time by a 
factor of four (roughly 50min -> 12min; yes, these serialized configure 
steps are extremely expensive on a 24-core build node).

Everything works well for me up to and including the `target-finalize` 
step. I have multiple CONFIG_TARGET_ROOTFS_*, and their rules clobber each 
other's auxiliary files, either directly (_fakeroot.fs), or via mkusers 
which creates duplicate rows in target/etc/group.

I tried to fix this by something like:

  .NOTPARALLEL: $$(BINARIES_DIR)/rootfs.$(1)

...but it just caused the entire build to once again become serial, 
package-after-package, which is not what I want.

Is there a better fix than running `make -j32 target-finalize && make` as a 
workaround?

With kind regards,
Jan