buildroot

dhrystone: remove usage of FOO_DIR_PREFIX

As the semantics of FOO_DIR_PREFIX will change in the following patch, we
want to remove unnecessary usage of this variable.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>