kconfig: allow long lines in config file

For some config options (CONFIG_EXTRA_FIRMWARE, for example), the length
of a config file line can exceed the 1024 byte buffer.

Switch from fgets to compat_getline to fix. compat_getline is an
internally implimented getline work-alike for portability purposes.

Signed-off-by: Cody Schafer <cody@linux.vnet.ibm.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>