proxmox-boot: read only first line of /etc/kernel/cmdline

proxmox-boot: read only first line of /etc/kernel/cmdline

following the commit of removing the wrong indentation of the linux
and initrd lines - this commit strips empty lines (and leading
trailing whitespace) in /etc/kernel/cmdline.

I managed to reproduce the issue reported in the forum [0] by adding
empty lines to /etc/kernel/cmdline) - without this - systemd-boot
booted quite happily even with the indentation.

considered using perl -pe with multiline matching but thanks to
Thomas' suggestion went with the shell-builtin read.

the check for existance of 'root=' in the resulting CMDLINE was added,
since my test-system had an empty line in the beginning, which again
rendered it unbootable.

quickly tested on a VM.

[0]: https://forum.proxmox.com/threads/problem-with-proxmox-boot-tool.99043/

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>