classes: Drop ';' delimiter from ROOTFS/IMAGE*COMMAND variables

classes: Drop ';' delimiter from ROOTFS/IMAGE*COMMAND variables

Originally these were shell functions but they have long since been processed by
bb.build.exec_func(). Since we no longer need shell syntax, we can drop the ';'
delimiters and just use a space separated string.

This cleans up the variable and quietly removes any stray ';' that do happen to
still make it in.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>