pkg-cmake.mk: add PATH in the configure command environment

Because BR_PATH is not exported in the environment beforehand running
cmake, it is necessary to add it on the cmake configure command.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>