core: don't reset MAKEOVERRIDES when re-entering make

We reset MAKEOVERRIDES to avoid passing down variables that are
overridden on the command line to the package build systems. Indeed,
the variables overridden on the command line will be Buildroot
variables and not relevant to the package build system. In particular
the O option is used by some packages and the value passed in on the
command line is plain wrong for the individual package.

However, in commit 916e614b, MAKEOVERRIDES was moved earlier and it
was reset _before_ re-entering make in the cases when something has
to be fixed up (incorrect umask, non-absolute paths in O or CURDIR).
Therefore, if make is re-entered, any command line overrides are lost.

This particularly bites the autobuilders, because they use
O=<relative path> to specify the output directory, and they add
BR2_JLEVEL=... to avoid starting too many jobs in parallel. The
BR2_JLEVEL override is lost.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>