toolchain: add support for external toolchain profiles and download

Instead of letting the user define all the details of his external
toolchain, we define a set of profiles for well-known external
toolchains (CodeSourcery ones only at the moment, can easily be
extended with other toolchains).

Once a profile has been choosen, the user is offered the choice of
either letting Buildroot download and install the external toolchain,
or (as before) to tell Buildroot where the toolchain is installed on
the system.

We of course provide a "custom profile", through which the user can
configure Buildroot to use a custom external toolchain for which no
profile is available.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>