docs/manual: do not override BR2_EXTERNAL

Currently, we forcibly override BR2_EXTERNAL when building the manual,
so as to avoid referring to packages therein from the manual.

However, when generating the lists of packages, we limit ourselves to
scanning for packages in (Buildroot's) TOPDIR and never search in
BR2_EXTERNAL. So, we do not really need to override BR2_EXTERNAL when
generating the manual to achieve the same result.

Furthermore, we're only looking for packages that are defined in the
sub-menu "Target packages", and never anywhere else, and especially not
in the sub-menu "User-provided options".

Finally, we're soon to completely eliminate use of BR2_EXTERNAL in
Kconfig altogether, so that would no longer have any impact anyway.

So, just leave BR2_EXTERNAL alone when generating the manual.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Samuel Martin <s.martin49@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>