cooker: Handle multiconfig name mappings correctly
"bitbake mc:arm:bash mc:arm:busybox"
works but
"bitbake multiconfig:arm:bash multiconfig:arm:busybox"
does not. The reason is the list is modified whilst iterating.
Don't do that.
[YOCTO #13607]
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>