build: Evaluate YAML_LIBS and LZMA_LIBS lazily

No need to check for a package that is only needed for a few targets.

BUG=chromium:683381
TEST=no more "Package yaml-0.1 was not found in the pkg-config search
path." when emerge'ing sys-boot/coreboot

Change-Id: I56cacea59d9ac13ee3ba587e475e1ff451e535a9
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1564472
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
1 file changed