stand/libofw: make OF_hasprop() part of the library

[mhorne]

Summary

Currently it is only needed by powerpc ofwfdt.c, and defined statically
there. Make it available as part of libofw, mirroring what we have in
the kernel.

Two small tweaks are made to the implementation:

1. Return type is changed to bool
2. Return 'true' when OF_getproplen() == 0. This matches the expected semantics of the kernel version, described in OF_hasprop(9).

[adrian]

Looks good; do we have a manpage with the other OF_ methods?

[mhorne]

In D56429#1291890, @adrian wrote:

Looks good; do we have a manpage with the other OF_ methods?

Yes, they are all in one, OF_getprop.9. I did not look in depth for what might be missing.

[adrian]

ah it's already there. yay!