fai-cd: drop dead code
lib/task_sysinfo: make sure device is a valid block device before acccessing it

Fails with:

| stat: cannot stat `/dev/sd?': No such file or directory
| /usr/lib/fai/task_sysinfo: line 65: [: =: unary operator expected

when using fai-cd on Debian/wheezy.
conf/NFSROOT: use grub-pc instead of grub package

The grub package is pointing to legacy GRUB which at least
on Debian/wheezy results in a failing fai-cd.