Drop PRIORITY variable

Drop PRIORITY variable

As discussed on the mailing list, this variable isn't useful and if wanted
would be better implemented by distros using pn-X overrides.

This patch executes:

find . -regex ".*\.\(bb\|inc\)$" | xargs sed -i '/^PRIORITY = ".*"$/d'

against the tree removing the referenced. Thanks to Phil Blundell for
the command.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>