docs/manual: do not hardcode name of the generated document

docs/manual: do not hardcode name of the generated document

Currently, GENDOC_INNER hard-codes some variables to have the name of the
document in them:
    MANUAL_$(2)_ASCIIDOC_CONF
    MANUAL_$(2)_ASCIIDOC_OPTS
    MANUAL_$(2)_A2X_OPTS
    MANUAL_$(2)_INSTALL_CMDS
    ...

Also, it defines some dependency on the generation rule, onto:
    manual-check-dependencies
    manual-check-dependencies-$(3)
    manual-prepare-sources

This is problematic, as it is not possible to have another document
generated with the GENDOC infra, or it would trigger the rules, and use
the variables for our own document, 'manual'.

Add a new argument to GENDOC_INNER to be the uppercase name of the
document, much like it is done for the PKG_*_INNER functions. Replace
any reference to 'manual' with the currently passed named of the current
document, $(1).

The remaining references to 'manual' are on purpose, as they really
pertain to our manual.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>