externalsrc.bbclass: Avoid symlink clashes for virtclasses

externalsrc.bbclass: Avoid symlink clashes for virtclasses

There was a race condifion in externalsrc_configure_prefuncs when the
same source folder is used for several variants of the same recipe,
like this:

EXTERNALSRC_pn-foo = "..."
EXTERNALSRC_pn-foo-native = "..."

The symlinks were created once for each variant of the recipe, and
where they led in the end depended on which do_configure task executed
last.  Create one set of symlinks for each variant by adding an EXTSRC_SUFFIX
variable to the end of the link names.
Tries to handle all known virtclasses and multilib variants.

Use a lockfile for externalsrc_configure_prefuncs to protect the
.git/info/exclude file.

Signed-off-by: Ola x Nilsson <olani@axis.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>