Makefile: add PATH to the USER_HOOKS_EXTRA_ENV

This can be useful for post-{build,image} scripts, in case some host-tools
were specifically built to be used by these scripts.

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