wic: accept '-' in bitbake variables
'-' is valid and common in bitbake variables (e.g. 'FOO_pn-bar'). Accept
it and other characters when reading the .env file.
Also, allow variables to be empty.
(From OE-Core rev:
e688ac8e92d2bc451d8b2d437596f630bedccd2c)
(From OE-Core rev:
2a69250abf61e51f633033ddb672e8f459191899)
Signed-off-by: Enrico Scholz <enrico.scholz@ensc.de>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
