SystemConfig: allow reading sku specific props

Sometimes, very similar devices share the same exact images but use
slightly different hardware. In this case, they distinguish themselves
with skus like ro.boot.product.hardware.sku. This SKU is also used to
distinguish between which HALs are exposed in the VINTF manifest.

In this CL, we add the following locations to read from:
odm/etc/sysconf/sku_${sku}/*.xml
odm/etc/permissions/sku_${sku}/*.xml

Only the configurations already available to be set from the ODM image
can be set here.

Bug: 119129238
Test: boot
Test: manually use unavailable-feature from odm sku directory

Change-Id: I465ac818e5c68f1118668f13b45940fd8fa0fa62