classes/qemuboot: add depends on qemu-system-native and qemu-helper-native

classes/qemuboot: add depends on qemu-system-native and qemu-helper-native

Any image that inherits qemuboot must also add image dependencies on
qemu-system-native and qemu-helper-native, otherwise the image won't
be able to be booted.

Currently this is done by conf/machine/include/qemu.inc, but not every
machine that uses qemuboot includes that file.

Move the EXTRA_IMAGEDEPENDS from qemu.inc into qemuboot.bbclass, so that
the dependencies don't have to be duplicated.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>