autoconf: add AUTOHEADER variable

autoheader is used by autoreconf, therefor we need to give the right
path to this tools for host/target autoreconfiguration.

Signed-off-by: Lionel Landwerlin <llandwerlin@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>