Makefile: internally use absolute paths to BR2_EXTERNAL

Using a relative path for BR2_EXTERNAL, and using an external defconfig,
such as in (from a Buildroot top-dir):
    make O=.. BR2_EXTERNAL=.. foo_defconfig

is broken. It is unclear why the %_defconfig rule recurses in that case.

This patch internaly makes BR2_EXTERNAL canonical (ie. makes it an absolute
path), and checks the directory exists.

[Peter: s/relatively/relative/ as suggested by Thomas]
Reported-by: Jérémy Rosen <jeremy.rosen@openwide.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Romain Naour <romain.naour@openwide.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>