bbb/fwtool.config: Fix typo in rootfs count

The rootfs block count was too big. This didn't break anything since the
rootfs is only partially used, but it should be right. fwtool currently
doesn't validate that there's no overlap between partitions.
ag150: add x86 support and ag150 target

Many common files were refactored to avoid duplication with the
Beaglebone Black.
Updates to support building out of tree

This cleans up paths so that building in Erlang project directories
works for x86. There are still hardcoded things to the Beaglebone is the
naming, but this fixes the functional aspects.
Enable ext4 in base ag150 image