configure: preserve built qmake and swap with native one

Let configure script build the real qmake, but right after it's built, swap
it with a native qmake for further internal use, preserving the real one.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
configure: bump path length from 256 to 512 characters

Increase the path length that gets hardcoded into generated config.cpp file
from 256 to 512 characters, as nativesdk path can be quite long.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>