installer: Fix reading /proc/cmdline when launched by GRUB

installer: Fix reading /proc/cmdline when launched by GRUB

The installer was reading the kernel command line and was looking for
certain values which configured the installer.

GRUB appended a trailing newline character which was not accounted for
and caused that the last parameter was not correctly compared to the
list of possible keys.

Fixes: #12656 - core 157: unattended installation don't work as expected on EFI
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>