doc: remove dependency on findutils on FreeBSD

doc: remove dependency on findutils on FreeBSD

Standard "find" on BSD does not support the "-printf" so gfind from
findutils package was used to enable full doc builds. We can remove this
extra dependency by using "sed" and "tr" to adjust the output from
regular find instead.

Fixes: 8260f4f98cfe ("mk: use script to generate examples.dox")
Fixes: 499fe9dfcfc7 ("doc: add dependency on examples for API doxygen")
Fixes: 897e55c8d27f ("doc: fix Doxygen examples build on FreeBSD")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>