Makefile: introduce BR2_PATH

Since the HOST_PATH and TARGET_PATH variables almost contain the same
things, let's factorize this in a single BR2_PATH.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Makefile: add $(HOST_DIR)/sbin to BR2_PATH

Extend BR2_PATH because few host-packages install programs in this
location.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
*.mk: replace (TARGET|HOST)_PATH by BR2_PATH

Signed-off-by: Samuel Martin <s.martin49@gmail.com>

---
changes v1 -> v2:
- make substitution of new occurenrces (pkg-python.mk and sdl.mk)
infra: remove unused {TARGET,HOST}_PATH definition

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
pkg-cmake.mk: add PATH in the configure command environment

Signed-off-by: Samuel Martin <s.martin49@gmail.com>

---
changes v1 -> v2:
- remove explicit PATH=... in configure commands in pkg-autotools.mk
  because it is already added via *_CONFIGURE_OPTS (ThomasP), and
  reword the commit message.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Makefile: export PATH

This can be useful for:
- package using custom build system that are not aware that the host
  tree is in $(O)/host/;
- 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>

---
changes v1 -> v2:
- extend commit message