Allows to quickly build systemd-boot by itself without having to
reference the full path of the build target:
$ ninja -C foo systemd-stub
ninja: Entering directory `foo'
[21/21] Generating src/boot/efi/linuxx64.efi.stub with a custom command
$ ninja -C foo systemd-boot
ninja: Entering directory `foo'
[10/10] Generating src/boot/efi/systemd-bootx64.efi with a custom command