Allow custom output_name

[raldone01]

I use refind and want my /boot to look like this:

[root@Elizabeth ~]# ls /boot/
 EFI					    initramfs-linux-git.img			   initramfs-linux-zen.img   vmlinuz-linux		     vmlinuz-linux-intel-lts-sriov.efi	 vmlinuz-linux-zen
'System Volume Information'		    initramfs-linux-intel-lts-sriov-fallback.img   initramfs-linux.img	     vmlinuz-linux-drm-tip-git	     vmlinuz-linux-intel-lts-sriov.png	 vmlinuz-linux-zen.efi
 devtree				    initramfs-linux-intel-lts-sriov.img		   intel-ucode.img	     vmlinuz-linux-drm-tip-git.efi   vmlinuz-linux-lts			 vmlinuz-linux-zen.png
 drivers				    initramfs-linux-lts-fallback.img		   linux.efi		     vmlinuz-linux-drm-tip-git.png   vmlinuz-linux-lts.efi		 vmlinuz-linux.efi
 initramfs-linux-drm-tip-git-fallback.img   initramfs-linux-lts.img			   loader		     vmlinuz-linux-git		     vmlinuz-linux-lts.png		 vmlinuz-linux.img
 initramfs-linux-drm-tip-git.img	    initramfs-linux-next-git-fallback.img	   refind_linux.conf	     vmlinuz-linux-git.efi	     vmlinuz-linux-next-git		 vmlinuz-linux.img.png
 initramfs-linux-fallback.img		    initramfs-linux-next-git.img		   refind_local.cer	     vmlinuz-linux-git.png	     vmlinuz-linux-next-git.efi		 vmlinuz-linux.png
 initramfs-linux-git-fallback.img	    initramfs-linux-zen-fallback.img		   rescue.sh		     vmlinuz-linux-intel-lts-sriov   vmlinuz-linux-next-git.png		 vmlinuz-linux.save.efi

Note: I will move the unsinged files to a secure filesystem soon.

By default sbupdate appends -signed and does not prepend vmlinuz- to fix this I added the following to /etc/sbupdate.conf:

# Return output file path corresponding to an image
#   $1: configuration name
function output_name() {
  echo "${ESP_DIR}/${OUTPUT[$1]:-${OUT_DIR}/vmlinuz-$1.efi}"
}

This workaround works but feels rather hacky.
Official naming scheme support would be nice.
If my current solution is fine then I can offer to add a section to the readme and document it as a feature.