Low: RA: SysInfo: use awk instead of bash to calculate memory and disk sizes

The initial motivation for this work was to remove bashisms.
However it also simplifies the code and fixes some correctness issues.

Signed-off-by: Simon Horman <horms@verge.net.au>

Mercurial revision: ea60c510840fa7e8b21bb34ba67de0b1730076e7