Move platform build scripts to packaging repo

Most platforms build scripts were moved to the packaging repo some years
ago, with only the Windows platform remaining.

All remaining code in the platform directory has now been moved and
merged into the packaging repo for consistency. (The cross compile
scripts were also updated to work with the new directory structure and
latest MythTV)

This commit removes the platform directory and therefore the
corresponding duplicate code.

Merged packaging commit:
MythTV/packaging@a969adc