Bug#465235: unqualified update-grub breaks install of older kernel packages

[Drake Diedrich]

We had an issue last week where existing kernel packages were
installing, but quietly failing to run update-grub.  It turned out to
be that the build environment for the kernels was an older
distribution and the resulting kernel packages had a test -x $grubpath
in the generated postinst.  The new change by grub-installer to
dequalify the update-grub path has the side effect of subtly breaking
the expected behavior of older kernel packages, which people may have
custom compiled for specific machines.  Or in our case, the compile
environment is the oldest distribution we support, and even though the
packages are new, the software to build them is older.  Rather than
just document the issue, it would be nice to address it, either with
continued generation of correct, fully-qualified paths in
/etc/kernel-img.conf or by causing a configure failure when the older
kernels are installed on a machine with a dequalified update-grub in
kernel-img.conf.

-Drake