busybox: Add support for busybox-init

busybox: Add support for busybox-init

in config metadata we can configure busybox based init and device
initializer ( mdev ) using e.g.

VIRTUAL-RUNTIME_dev_manager = "busybox-mdev"
VIRTUAL-RUNTIME_login_manager = "busybox"
VIRTUAL-RUNTIME_init_manager = "busybox"
VIRTUAL-RUNTIME_initscripts = "initscripts"
VIRTUAL-RUNTIME_keymaps = "keymaps"
DISTRO_FEATURES_BACKFILL_CONSIDERED += "sysvinit"

busybox can be used to provide init system
combined with mdev it makes it a complete init
system for really tiny systems.

This patch uses above defines to configure features in busybox to enable
the init system and mdev in a configurable manner

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>