boot/barebox: fix target bareboxenv command compile

boot/barebox: fix target bareboxenv command compile

The buildroot custom bareboxenv compile command misses the additional
include path 'scripts/include' to gain access to the local copy of the
kernel header files (which leads to compile error when using an older
toolchain).

This could be fixed by enhancing the custom bareboxenv compile command
(see [1]) or by using the barebox build system by simply enabling the
CONFIG_BAREBOXENV_TARGET option (available since April 2012, see [2])
instead (as suggested by Yann E. MORIN).

Fixes (with BR2_TARGET_BAREBOX_BAREBOXENV enabled):

  build/barebox-2019.12.0/scripts/bareboxenv.c:100:10: fatal error: linux/list.h: No such file or directory

[1] http://lists.busybox.net/pipermail/buildroot/2020-January/270942.html
[2] https://git.pengutronix.de/cgit/barebox/commit/?id=afb03d7a554a2911a3742e316f011319fcb416f1

Note: a user who would previously provide a barebox config file which
had CONFIG_BAREBOXENV_TARGET=y, but a Buildroot config file which did
not have BR2_TARGET_BAREBOX_BAREBOXENV=y, would have bareboxenv-target
built, but it would not be installed in the target. Now, and unset
BR2_TARGET_BAREBOX_BAREBOXENV will not even build it, but his is not a
regression: it was anyway previously not installed.

Reported-by: Frederick Gotham <cauldwell.thomas@gmail.com>
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
[yann.morin.1998@free.fr:
  - also explicitly disable it when not selected
  - rewrap commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>