cli: Fix detection of incorrect kernel

When rebooting into a new kernel, "fastpath sut configure" checks that
the booted kernel image is the one it asked for. The booted kernel image
is added to the kernel cmdline by grub. But it turns out that different
systems report the directory structure differently. My primary test
system reported "/<kernel-name>", but another system reports
"/boot/<kernel-name>" (both are mounted at /boot once linux is booted).
The regex was not able to deal with the latter and therefore we
concluded that the incorrect kernel was booted.

Fixes: 27aae23a ("cli: Implement "fastpath sut configure" command")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>