Support retrieving WCNSS_qcom_wlan_nv.bin from system partition
I've tested this change on my samsung-a5 running Arch Linux ARM. I'm able to bring wifi up successfully with this patch and an intact system partition.
I haven't tested it on pmOS, but I'm 98.62% certain that it won't break anything there, since:
- the FW_PARTITIONS scanning/mounting logic is already robust
- it prioritizes retrieving
WCNSS_qcom_wlan_nv.bin
from persist partition and will only look for it in system as a fallback and - the existing symlink farm code is already in place to prevent overwriting firmware that already exists.