meson: support custom led yaml path

Most machines using phosphor-led-manager push their led.yaml with
virtual/phosphor-led-manager-config-native, which installs led.yaml
to a native tree.
Without led yaml path support, those machines are getting the wrong
led groups.
For compatibility, add this support back.

Signed-off-by: Seires Li <seiresli@pcpartner.com>
Change-Id: I73aced08a288db5d70440e0c18d194a30158e467
2 files changed