fix buildroot sed handling on Mac OS X, try #2

Hello!

Here's a series of patches (ultimately) intended to make buildroot work on Mac OS X. This first series tries to fix sed handling. The OS X (and BSD) sed are different from the implicitly assumed GNU sed, therefore it makes sense to build GNU sed for the buildroot toolchain. The current code in buildroot does not correctly determine that the OS X sed does not work for some of the required cases, and the existing sed package cannot build sed on OS X.

This is try #2; fixed a wrong absolute path and handling of the case where the host has a working gsed or gnused.

-- Heikki Lindholm