Makefile: rename USER_HOOKS_EXTRA_ENV to EXTRA_ENV

This variable contains extra environment variables that we can not export
since they are clashing with some build systems (eg. BUILD_DIR with
u-boot).

So, we may need these variables for uses other than the user's hooks
for instrumentation. For example, we'll use them later on to export
BUILD_DIR to the download helper scripts.

Fix comment, too.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>