parse: make vars_from_file return None for non-recipes

It doesn't really make sense to set PN from .conf files, for example.
More concretely, this avoids the config hash changing unnecessarily
within Hob due to PN effectively changing (since bblayers.conf is
parsed first and then .hob.conf).

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
build: fix handling of task override for tasks with underscores in their names

Tasks whose names contain underscores (such as do_populate_sdk in OE)
when converted to a task override do not function properly. If we
replace underscores with hyphens we can still have a working override
for these tasks.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>