main/postmarketos-mkinitfs: set pmOS-specific firmware path

Currently we rely on the fact, that the kernel asks user space to load the firmware for it, so we can use the /lib/firmware/postmarketos directory for firmware.

As the kernel has support for loading the firmware itself since v3.7, and the parameter for adding an additional path to the search paths was added with v3.10, we can use that to not rely on CONFIG_FW_LOADER_USER_HELPER being set.