Use an auxiliary variable to record configured ram size for npe-400.

When ram is over 256 MiB, npe-400 uses the excess as iomem. The ram
size is limited to 256 MiB because there are devices mapped to the
256-512 MiB address range.

Option 1: implement c7200 set_iomem and limit the ram to 256 MiB
(not backward compatible, only npe-400 would support set_iomem)
Option 2: use an auxiliary variable for the ram size (hackish)
Option 3: separate config options from actual runtime values (complex)

I picked option 2.

Fixes #49 - IOS crashes after router restart