yocto_kernel: check current items before add a new one

yocto_kernel: check current items before add a new one

When use "yocto-kernel config add" to add the same config many times,
all of these are list when use "yocto-kernel config list" to check.

This fix modify routine yocto_kernel_config_add, if the new added
components already exist in current configuration, just igore them.
Now, one config could only be added one time.

[YOCTO #4558]

(From meta-yocto rev: 655ccc5ed77b52fb62dab5f6cfdf3de39b1bf055)

Signed-off-by: Ning Zhang <ning.zhang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>