qemu-x86: Use config_distro_bootcmd

qemu-x86: Use config_distro_bootcmd

Converts qemu x86 machines to boot using distro_config. The intent is to
allow u-boot in qemu to be maximally compatible with many boot methods
without having to change the config. Previously, u-boot would only boot
in a very limited set of circumstances where there was a /boot/vmlinuz
on scsi 0:3 with no ramdisk.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: convert doc updates to reST]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>