bitbake: BBHandler: Fix addtask and deltask

bitbake: BBHandler: Fix addtask and deltask

The following commands are not supported, but they were ignored silently, that
may suprise users:

* addtask task1 task2
  task2 is ignored

* addtask task1 before task2 before task3
  Should be: addtask task1 before task2 task3

* addtask task1 after task2 after task3
  Should be: addtask task1 after task2 task3

* deltask task1 task2
  task2 is ignore

This patch can check and warn for them.

[YOCTO #13282]

(Bitbake rev: 675689aa7cc7287efecf8ef775ca2059369167f1)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>