buildroot

core: re-enter make if $(CURDIR) or $(O) are not canonical paths

When $(CURDIR) and/or $(O) contain symlinks in their paths, they can be
resolved differently, depending on each package build-system (whether it
uses the given paths or get the absolute canonical ones).

Using absolute canonical paths will help achieving reproducible builds and
will make easier tracking down host machine paths leaking into the host,
target or staging trees.
So, this change ensures the build takes place with the CURDIR and O
variables are set to their absolute canonical paths.

In order to recall the toplevel makefile with absolute canonical paths
for $(CURDIR) and $(O), we need to:
1- Compute the absolute canonical paths for $(CURDIR) and $(O) that will
   be passed to the sub-make. This is achieved using the 'realpath' make
   primitive. However, some care must be taken when manipulating O:
   - the out-of-tree makefile wrapper happens a trailing "/.", we need
     to strip this part away to not break the comparison driving the
     sub-make call;
   - the user can leave a trailing '/' to $(O);
   - according to [1,2], realpath returns an empty string in case of
     non-existing entry. So, to avoid passing an empty O= variable to
     sub-make, it is necessary to define the output directory and create
     it prior to call realpath on it (because on the first invocation,
     $(O) usually does not yet exists), hence the trick doing the mkdir
     right before calling realpath.
2- Update EXTRAMAKEARGS with the absolute canonical $(O) and use it
   when call recalling the top-level makefile with umask and paths
   correctly set.
3- Lastly, update the condition for setting the CONFIG_DIR and
   NEED_WRAPPER variables.

Note:
* This change takes care of the makefile wrapper installed in $(O) to
  avoid unneeded make recursion.

[1] https://www.gnu.org/software/make/manual/html_node/File-Name-Functions.html
[2] http://man7.org/linux/man-pages/man3/realpath.3.html

Reported-by: Matthew Weber <matt@thewebers.ws>
Cc: Matthew Weber <matt@thewebers.ws>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>