bustle: use TARGET_MAKE_ENV instead of setting PATH in the make environment

TARGET_MAKE_ENV already contains the PATH definition among other useful
variables.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

---
changes v4 -> v5:
- rebase
- add Acked-by tag

changes v3 -> v4:
- add this change to the series (ThomasP)
Makefile: export PATH including the Buildroot host bindirs

This patch exports the PATH variable containing the factorized content
of TARGET_PATH and HOST_PATH (because they were fairly similar).

TARGET_PATH and HOST_PATH are now set using only the PATH variable.

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

---
changes v4 -> v5:
- make my call: export PATH
  - merge patch 7/7 into this one, so:
  - do not use intermediate BR_PATH variable, just export PATH with th
    right content
  - now, HOST_PATH and TARGET_PATH are just $PATH
- remove unrelevant comments (Arnout)

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

changes v2 -> v3:
- rebase

changes v1 -> v2:
- rebase
Makefile: add $(HOST_DIR)/sbin to the PATH

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

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

---
changes v4 -> v5:
- rebase
- extend PATH instead of BR_PATH

changes v3 -> v4:
- rebase
- fix typo in commit log (ThomasP)
- rename BR2_PATH -> BR_PATH

changes v2 -> v3:
- rebase

changes v1 -> v2:
- rebase
*.mk: remove all occurences of TARGET_PATH and HOST_PATH

Thanks to the 2 previous patches, the PATH now contains all locations
in which host-packages may install programs.

This patch removes the occurrences TARGET_PATH and HOST_PATH from the
*.mk files.

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

---
changes v4 -> v5:
- rebase
- remove all occurence of {TARGET,HOST}_PATH
- fixup a couple of PATH definitions

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)