kernel-install/90-loaderentry.install: fix cmdline parsing

kernel-install/90-loaderentry.install: fix cmdline parsing

A recent commit (2f3a215) changed the parsing of /proc/cmdline to use a
shell array. Unfortunately, this introduced a bug: "read -ar line"
populates the shell variable $r, not $line. This breaks installation of
new loader entries:

  # kernel-install add 3.17.1-304.fc21.x86_64 \
      /boot/vmlinuz-3.17.1-304.fc21.x86_64
  Could not determine the kernel command line parameters.
  Please specify the kernel command line in /etc/kernel/cmdline!

This commit alters the read command to correctly populate the $line
array instead.