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>

meta/classes/systemd-boot.bbclass

@@ -4,9 +4,7 @@
 
 # systemd-boot.bbclass - The "systemd-boot" is essentially the gummiboot merged into systemd.
 #                        The original standalone gummiboot project is dead without any more
-#                        maintenance. As a start point, we replace all gummitboot occurrences
-#                        with systemd-boot in gummiboot.bbclass to have a base version of this
-#                        systemd-boot.bbclass.
+#                        maintenance.
 #
 # Set EFI_PROVIDER = "systemd-boot" to use systemd-boot on your live images instead of grub-efi
 # (images built by image-live.bbclass or image-vm.bbclass)