ueventd: let scripts provide firmware directories

Since some vendors will have firmware in mount points in
/mnt/vendor/..., we extend the ueventd script language to allow
specifying the firmware directories.

Also, move the existing 4 directories to ueventd.rc as a primary user
of this mechanism.

Bug: 111337229
Test: boot sailfish; firmwares load
Change-Id: I0854b0b786ad761e40d2332312c637610432fce2