[FIX] fix efi provider
To make the first build has been noticed that in the qemu bbclass the EFI_PROVIDER was set to grub-efi-seco that, without an override, doesn't change. Indeed, in the case where the IMAGE_CLEA_OS_BOOT_FILES should add boot/*, it doesn't. This has been fixed by adding to the qemu bbclass a machine specific EFI_PROVIDER such that, even if the bbclass is parsed every time, the assegnation of the variable is done when the machine is qemux86 or qemux86-64
Signed-off-by: carlo carlo.ferriolo@seco.com