make arch and variant parsing more robust

Post-process the arch and cpu variants to treat the arch name or
"generic" as an empty variant.  Filter out extra empty abis.  Ignore
empty arches.  Print a useful error message when appending properties
fails to find the target field, when an unknown architecture is used, or
when a toolchain has not been implemented for the selected
architecture.

Change-Id: I671d4cd04975f4f29aefc4267b3a624868ce6a75
2 files changed