build: BSD compile fixes

following patch allows to build images for Qemu ARM on
OpenBSD 5.2 amd64 and FreeBSD 9.1 amd64.

Mostly small pieces of code changes to get things right on the
specific platform.

Updated the README to describe better, which tools on the host
are required. Added some kind of prepare scripts to install needed
tools on BSD via packages.

Signed-off-by: Waldemar Brodkorb <mail@waldemar-brodkorb.de>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35900 3c298f89-4303-0410-b956-a3cf2f4a3e73