*.mk: replace (TARGET|HOST)_PATH by BR_PATH

Thanks to the 2 previous patches of the series, BR_PATH contains
all locations in which host-packages may install programs.

This patch replaces the occurrences TARGET_PATH and HOST_PATH with
BR_PATH, everywhere these variables are used in the *.mk files.

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

---
changes v3 -> v4:
- rebase
- fix typo in commit log (ThomasP)
- rename BR2_PATH -> BR_PATH
- make substitution of new occurenrces (gpm.mk)

changes v2 -> v3:
- rebase
- no one-line commit log (ThomasP)

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

Since the variables TARGET_PATH and HOST_PATH are not used anymore,
let's remove them.

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

---
changes v3 -> v4:
- rebase
- rename BR2_PATH -> BR_PATH

changes v2 -> v3:
- rebase
- no one-line commit log (ThomasP)

changes v1 -> v2:
- rebase
pkg-cmake.mk: add PATH in the configure command environment

Because BR2_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>

---
changes v3 -> v4:
- rebase
- rename BR2_PATH -> BR_PATH

changes v2 -> v3:
- rebase
- no one-line commit log (ThomasP)

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.
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 v3 -> v4:
- rebase
- rename BR2_PATH -> BR_PATH

changes v2 -> v3:
- rebase
- extend commit loghanges v2 -> v3:

changes v1 -> v2:
- extend commit message