sepolicy: Added context mount flag for firmware_file type .

Firmware partition is by default mounted as vfat with  selabel as
sdcard_external , To explicitly  reload the context based  on the
context flag passed in mount option  we need to have firmware_file
as contextmount_type .

Change-Id: Id5e35cad285f7835e45d9583e7f2f4f367631201