kernel: add bootargs support

Add support for passing args to main(). The
content of bootargs is taken from get_bootargs()
which should be implemented for each loader and
then its split into args and passed to main.

Signed-off-by: Jakub Michalski <jmichalski@internships.antmicro.com>
Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>