Support for memdisk (syslinux)

https://wiki.syslinux.org/wiki/index.php?title=MEMDISK

After adding memdisk.modules & memdisk.files to my custom /etc/mkinitfs/features.d/ folder, mkinitfs adds kernel modules to initramfs, but not /usr/bin/memdiskfind file, may it's just a local bug and will work in "upstream" (I don't think files need 777 permission here? Normally build user already has permissions for accessing memdiskfind... Otherwise this could be the bug, then I'll create a patch for syslinux). Other stuff is working fine on my local env :-)

PS: I'll also create a MR for aports to support memdisk-profile on-the-fly, so others don't need to waste their time searching for correct options :D