Add fetch_debs target to get prebuilt debs from x20 cache.

* Don't cache aiy*.deb
* Decreases total build time by over 2 hrs vs building all.

Usage from clean env:

Build with cached debs:
m fetch_debs
m

Build all including debs:
ROOTFS_FETCH_DEBS=false m

Push debs to cache (make sure they're all OK first):
ROOTFS_FETCH_DEBS=false m
ROOTFS_PUSH_DEBS=true m push_debs

Change-Id: I4a79b3d2d5643c050851f7e5cf8ab5977f076dc2
2 files changed