tools/power turbostat: print the kernel boot commandline

It would be handy to have cmdline in turbostat output. For example,
according to the turbostat output, there are no C-states requested.
In this case the user is very curious if something like
intel_idle.max_cstate=0 was used, or may be idle=none too. It is
also curious whether things like intel_pstate=nohwp were used.

Print the boot command line accordingly:
turbostat version 21.05.04 - Len Brown <lenb@kernel.org>
Kernel command line: BOOT_IMAGE=/boot/vmlinuz-5.16.0+ root=UUID=
 b42359ed-1e05-42eb-8757-6bf2a1c19070 ro quiet splash vt.handoff=7

Suggested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>