busybox: ship a symlink farm for ptest

busybox: ship a symlink farm for ptest

Ship a symlink farm for busybox, which correctly considers SUID
split. This ensures that all utilities used in busybox's test cases
will first use that ones that are provided by busybox.

Modify run-ptest to prepend the directory to PATH, and also change
variable name from current_dir to current_path, as the former is
a little misleading. `readlink -f $0' gets a path to the current
script instead of the current directory.

(From OE-Core rev: 7ef8f96941ed52b2a00cbe8f57511a8891b39698)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>