core: do not hard-code inclusion of br2-external in Kconfig

core: do not hard-code inclusion of br2-external in Kconfig

Move the inclusion of br2-external's Config.in to the generated kconfig
snippet.

This will ultimately allow us to use more than one br2-external tree.

Offload the "User-provided options" menu to the generated Kconfig
snippet. We can also move the definition of the Kconfig-version of
BR2_EXTERNAL into this snippet.

We introduce an extra check that was not present in the previous code,
to check that we do have permission on that directory. Prevciously, it
was handled as a side effect of not being able to cd into there, but it
is cleaner to check it expressly.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>