Changes

OBS: disable_manuals: adjust for osmocom-bb

With the related change, we build manauals in the debian package of
osmocom-bb. The osmocom:nightly:asan feed gets built with manuals
disabled. Fix that building the osmocom-bb package in the asan feed
currently fails with:

[  160s] /usr/bin/make -C doc/manuals
[  160s] make[2]: Entering directory '/usr/src/packages/BUILD/doc/manuals'
[  160s] OSMO_GSM_MANUALS_DIR: /usr/src/packages/osmo-gsm-manuals (fallback)
[  160s] ERROR: OSMO_GSM_MANUALS_DIR does not exist!
[  160s] Install osmo-gsm-manuals or set OSMO_GSM_MANUALS_DIR.
[  160s] Makefile:6: /build/Makefile.asciidoc.inc: No such file or directory
[  160s] make[2]: *** No rule to make target '/build/Makefile.asciidoc.inc'.  Stop.

Related: osmocom-bb Ib04160287c98814a6f2ae104197abda563bdd5b9
Change-Id: I88f3291acbf067ab8779b298e828e18eb828a511