Makefile: add BR_BUILDING variable

Some packages do some sanity checks on their configuration, for
example linux checks that the defconfig string is not empty when a
defconfig is used. Such checks are currently always performed, except
when the 'source' target is part of make goals.

This is problematic for two reasons:

 - Other targets such as 'source-check', 'external-deps' or
   'legal-info', that do not consist in doing a build, cannot be
   executed in such situations.

 - The current code removes the check as soon as one of the targets is
   source. But if there are other non-source targets called at the
   same time, the checks are ignored.

This commit therefore introduces an internal variable called
BR_BUILDING, which tells packages if we are actually building or
not. A variable nobuild_targets indicates the targets that we do not
consider as being build targets.

For the moment, nobuild_targets only contains 'source', to be
completely iso-functional.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>