[rfc] rework mkinage / live mode

as we provide only uefi boot images on releases.openelec.tv please consider the following:
-- I have not yet seen a x86_64 hardware where an uefi image doesnt boot. this PR forces "make image" to always produce efi (hybrid, they boot on non-uefi hardware, too) images. 
-- so far 'live' mode is available only when booting in legacy mode, but is quite useless because
  (1) /storage cant be resized at all on gpt partitioned images. this PR completely removes /storage on x86
  (2) /storage is only 32MB. this PR uses /storage as tmpfs on x86
-- (EDIT) I want vesamenu stuff removed. it caused lot of trouble since 3.0 (3.2?). I've helped countless of users with "hey lol its frozen at boot menu" issues to remove UI vesamenu.c32 fromm their syslinux.cfg.

my personal opinion that "live" mode should not be persistent. it's intended for a quick check if user's hardware (gpu, whatever) is supported, and whether OE can boot at all. with this PR there is "boot: " prompt shown, and one can type "live" in to boot in live mode. 

dont merge until this is tested and it's confirmed everything wotks on RPi / Cuboxi. 

// cc @vpeter4 I know you want persistent live mode. I'd appreciate your opinion here.