cli: Fix grub defaults key/value parser

_get_grub_defaults() reads /etc/default/grub and parses the contents
into key/value pairs. It previously did this by splitting on "=". But
this breaks if the value contains an "=". So instead let's find the
first "=" and split on that marker.

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