toolchain/toolchain-external-andes-nds32: new package

toolchain/toolchain-external-andes-nds32: new package

This commit adds a new package for the Andes external toolchain for
the nds32 Little Endian architecture.

https://github.com/vincentzwc/prebuilt-nds32-toolchain/releases/download/20180521/nds32le-linux-glibc-v3-upstream.tar.gz

Signed-off-by: Che-Wei Chuang <cnoize@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Nylon Chen <nylon7@andestech.com>
[Thomas:
 - rename .mk and .hash files to carry the proper package name
 - fix <pkg>_SITE variable, which was incorrect
 - add prompt in Config.in
 - add missing include of Config.in in toolchain/toolchain-external/Config.in
 - add missing selects for RPC and SSP, since the toolchain supports
   both
 - drop BR2_TOOLCHAIN_EXTERNAL_URL option, the toolchain URL is
   provided by the .mk file]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>