gummiboot: Remove/change gummiboot references with systemd-boot

gummiboot: Remove/change gummiboot references with systemd-boot

After systemd-boot was introduced, its been tested for a while with no major
issues being found until now, this patch completely replaces all gummiboot
instances with systemd-boot ones, taking the next step into cleaning
up systemd-boot/gummiboot.

[YOCTO #10332]

(From OE-Core rev: f9a61d3400ad9068a6d83b8eb6aefe3098c58e68)

Signed-off-by: Alejandro Hernandez <alejandro.hernandez@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>