barebox: set default BAREBOX_CONFIG for qemu machines

barebox: set default BAREBOX_CONFIG for qemu machines

These are set in the barebox class rather in the corresponding machines
(where they would belong otherwise) to keep the impact of barebox to
oe-core minimal for now.

"multi_v7_defconfig" is used for qemuarm since this is the default
barebox armv7 config that just enables all supported platforms.

"multi_v8_defconfig" is used for qemuarm64 sine this is the default
barebox armv8 config that just enables all supported platforms.

"efi_defconfig" is used for qemux86-64 which is the primary platform
where barebox will not be the first stage bootloader but an EFI payload.

Since these changes make barebox a provider for virtual/bootloader,
explicitly default to u-boot in the corresponding MACHINE configs to not
unnecessarily surprise users.

Signed-off-by: Enrico Jorns <ejo@pengutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>