Add additional check on /dev/nvram to avoid misleading error message

* ybin/ybin: Add additional check on /dev/nvram, if we cannot read a
  couple bytes report it as broken.  This avoids misleading
  `Incompatible nvsetenv' message on kernels without /dev/nvram support.
git-archimport-id: erbenson@alaska.net--public/yaboot--devel--1.3--patch-71
3 files changed