core/br2-external: fix use of relative paths

core/br2-external: fix use of relative paths

Fixes #9576

When the path to a br2-external tree is relative, make enters an endless
recursive loop (paths elided for brevity):

    $ make BR2_EXTERNAL=.. foo_defconfig
    make[1]: stat: ../configs/../configs/../configs[...]/toto_defconfig: Filename too long
    make[1]: *** No rule to make target '../configs/../configs/../configs[...]/toto_defconfig',
    needed by '../configs/../configs/../configs[...]/toto_defconfig'.  Stop.
    Makefile:79: recipe for target '_all' failed
    make: *** [_all] Error 2

It is a bit complex to understand the actual technical reason for this
never-ending expansion; it seems it happens in the code generated by the
percent_defconfig macro. Not sure why, though...

But the root cause is the relative path.

Just use absolute, canonical paths to br2-external trees. Always.

[Peter: add bugzilla reference]
Reported-by: outtierbert@gmail.com
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>