toolchain/wrapper: fill and reference info.mk in staging_dir ext toolchain

toolchain/wrapper: fill and reference info.mk in staging_dir ext toolchain

Currently when external toolchain is used, a info.mk is created (with
to-be-filled values) but is never actually filled with real values and
is never actually used in rules.mk.

This info.mk is used down the code with special packages like libgcc to
reference info like gcc version and package special library.

To mimic what is done with internal toolchain, add the include in
rules.mk for info.mk also for external library and in fix
toolchain/wrapper to fill the staging_dir info.mk.

The logic is to check if the external toolchain provide an info.mk and
if it doesn't we at least fill the GCC_VERSION with the value set in the
.config file.

With this special library like libgcc correctly reference and have set the
GCC_VERSION variable.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>