As the default package list is now handled using the 'packages' directive,
explicit wpad-mini removals in target definitions can be dropped.

This, together with a new try_config directive, will allow checking the
generic config and make it more flexible.

Fixes #1126

Don't stop after the first error, as later error messages may make the
cause of the error much clearer in some cases.

Setting DEVICES will also suppress cleaning the package directory,
improving incremental build speed.