toolchain-external: allow downloading a custom toolchain

This patch adds the possibility to download a custom external
toolchain, in addition to the existing support of preinstalled custom
external toolchains.

With the modified configuration, the user is presented with the
following options:
- Toolchain type: Buildroot toolchain | External toolchain | Ct-ng toolchain

In case of External toolchain:
- Toolchain: the CodeSourcery toolchains | Custom toolchain
- Toolchain origin: Toolchain to be downloaded and installed | Pre-installed toolchain

In case of Toolchain to be downloaded, the user is presented with:
- Toolchain URL
In case of Pre-installed toolchain, the users sees:
- Toolchain Path

For CodeSourcery toolchains, the toolchain URL field is not used (the
URLs are directly coded in ext-tool.mk).

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>