configure: Pull linux-headers/asm symlink creation out of loop

Pull the creation of the linux-headers/asm symlink out of the loop
so we don't pointlessly delete and recreate it once for each target.
Also move the setting of the includes variable up so that it is
in the same place as the other code which sets this variable.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
