Feature: Add support to configure an external ldconfig with menuconfig

Hi,

this feature could be helpful for people who build their image within a chroot of an "rolling release" distributions (e.g. ArchLinux). It would be a shame, to downgrade a package that much ;).

With my actual glibc v2.17 I'm not able to create the necessary symlinks for a different architecture, so I specify the variable 'BR2_EXTERNAL_LDCONFIG_PATH' with the path to an older package of glibc (v2.10) which has support for different architectures or is missing the feature for excluding different ones.

Maybe this feature could be merged in the future (isn't that essential for the most of the people).

Regards, JoeKar

PS: Sorry for the 5 commits, they were created while struggling with git. But only 2 files should have changed compared to master.