busybox: add the ability to split the busybox binary
This patch enables us to split the busybox into two binaries, one
containing suid applications, and the other containing nosuid apps.
Add a variable, BUSYBOX_SPLIT_SUID, to control whether to split the
busybox binary into two parts. We default it to "1" to enable the
splitting, but users could still override it to disable the splitting.
After all, busybox has no internal support for this suid apps splitting,
so there might be users out there who want just one busybox binary.
The basic idea here is to build the busybox twice, each with the correct
configuration items. We extract the non-app part of the original .config
file, and merge this part with the suid-app part to form a .config which
contains only suid apps. The same strategy applies to the non-suid apps.
