Build DEVICE manifests and fragments for each SKU

Single vendor can support multiple skus with different
capabilities. Add support to define and build sku
based device manifests.

Specify DEVICE manifests in BoardConfig.mk as below.

  # installed to /vendor/etc/vintf/manifest_{sku}.xml
  DEVICE_MANIFEST_SKUS := sku1 sku2 sku3

  DEVICE_MANIFEST_SKU1_FILE := device/foo/bar/manifest_sku1.xml
  DEVICE_MANIFEST_SKU2_FILE := device/foo/bar/manifest_sku2.xml
  DEVICE_MANIFEST_SKU3_FILE := device/foo/bar/manifest_sku3.xml

Bug: 148601741

Change-Id: Id57d4ad21b0d3edf8d60d07446b6b806c1df20ab
Merged-In: Id57d4ad21b0d3edf8d60d07446b6b806c1df20ab