msrtool: Read both MSR values from file in diff mode

msrtool: Read both MSR values from file in diff mode

Previously, msrtool would assume that MSR values should be compared
between stored value in file and current value in hardware which msrtool
was running on. This does not always fit the use case and with this
change msrtool can now compare two sets of MSR values stored in a file.
If only one MSR value is stored in the file, msrtool will behave as
before, and read the second MSR value from hardware.

This change means that msrtool does not always need access to the system
MSR functions so it can now be run as a regular user when using diff
mode with both MSR values stored in the file.

Signed-off-by: Peter Stuge <peter@stuge.se>
Acked-by: Stefan Reinauer <stepan@coresystems.de>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@5033 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1