Unfortunately, we cannot re-use the existing Makefile rules from:
$(OSMO_GSM_MANUALS_DIR)/build/Makefile.vty-reference.inc
because they do not allow to generate the list of $(DOCBOOKS) from
a template, and require the project to store everything in separate
folders with specific names. Also, those rules expect that the
target PDFs contain only a single word in their names (for example,
'osmoapp-vty-reference', not 'osmo-app-vty-reference'), while in a
project with multiple similarly named targets this would reduce
readability (imagine 'osmotrxuhd-vty-reference').
Change-Id: I798ea3b7417b8ca3e9c7d50911158c5413526237
Depends: I6aac73d998c5937894233631e654a160d5623198
Related: SYS#4937, SYS#4910
