cli: Fix mishandled chars in cmdline

It turns out that if the kernel cmdline has a semicolon (;), grub will
interpret that as a command separator when executing the boot-time
commands, and the result is that anything in the command line after the
semicolon is not passed to the kernel.

I can't find any official documentation for this behaivour or for which
other characters might be special. But by experiment, I've found that
escaping semicolon with a backslash will cause grub to do the right
thing. So let's just fix semicolon for now. We will have to fix anything
else that comes up as it arises.

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