fdatasync is an optional part of POSIX and mkimage uses the more general
fsync on Sun and FreeBSD as an alternative. Add macOS[1] and OpenBSD to the
list as well.

This is in-line with what U-Boot does since 31cbe80c33
("mkimage: fix compilation issues on OpenBSD").

[1]: macOS >=10.7 does indeed define (but not declare anywhere) a seemingly
     working fdatasync, but we won't consider this here.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>