data: Account for pre/postfunc functions when calculating dependencies

pre/postfuncs were not being added to checksums. This meant that when reconfiguration
occurred, tasks were not always being rerun when they should. This include
sstate functions as well as systemd's do_install function in the OE metadata.

With the addition of postfuncs, its possible a shell task can have a python
pre/postfunc so we have to guard against this when generating shell output
in emit_func.

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