nvramtool: reduce memory mapping

Instead of trying to map the first megabyte, only map what is
required to read the tables.

Change-Id: I9139dbc8fd1dd768bef7ab85c27cd4c18e2931b3
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/2485
Tested-by: build bot (Jenkins)
Reviewed-by: Peter Stuge <peter@stuge.se>
1 file changed