gendoc infra: move manual build location into $(BUILD_DIR)/docs/manual

gendoc infra: move manual build location into $(BUILD_DIR)/docs/manual

This patch reworks the manual source preparation by:
- moving the build directory under $(BUILD_DIR)/, this keeps consistency
  with the other Buildroot infrastructures;
- adding a couple of targets: 'manual-rsync' and 'manual-prepare-sources',
  to deal more efficiently with the manual sources and avoid rsync-ing
  them on every single manual-* target.

The 'manual-rsync' target only copies the manual sources under git, while
the 'manual-prepare-sources' also takes care of the generated ones. These
targets are now run only once,  and the manual build is no longer cleaned
after each manual format generation.

Now, the 'manual-clean' target only remove the manual build directory, but
keeps the output one $(O)/output/doc/manual unchanged.

Doing so (moving the manual build directory and keeping it between 2
manual format generation) ensures that all generated sources are taken
in account when generating the manual [1].

[1] http://lists.busybox.net/pipermail/buildroot/2014-August/104421.html

Cc: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr: moved into $(BUILD_DIR)/docs/manual as per
Thomas P. suggestion]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>