net: Add the BOOTP_DNS2 option to Kconfig

Commit 3b3ea2c5 ("Kconfig: cmd: Make networking command dependent on NET")
removed the help documentation from the README but didn't add it back to Kconfig.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Duncan Hare <dh@synoia.com>