Decrease network file copy time in driver library

To decrease the time it takes to construct a network instance directly
from a file, the driver library now memory maps the file instead of
copying it before handing the data to the Network UAPI.

Change-Id: Iff653555cca357786ef338063870c5195177239c
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>