core: allow br2-external trees to provide pre-configured toolchains

core: allow br2-external trees to provide pre-configured toolchains

Since we have a choice for the pre-configured pre-built toolchains,
there is no possbility for a br2-external to provide its own. The
only solution so far for defconfigs in br2-external trees is to use
BR2_TOOLCHAIN_EXTERNAL_CUSTOM and define all the bits by itself...

This is not so convemient, so offer a way for br2-external trees to
provide such pre-configured toolchains.

To allow for this, we now scan each br2-external tree and look for a
specific file, provides.toolchains.in. We generate a kconfig file that
sources each such file, and that generated file is sourced from within
the toolchain choice, thus making the toolchains from a br2-external
tree possible and available in the same location as the ones known to
Buildroot:

    Toolchain  --->
        Toolchain type (External toolchain)  --->
        Toolchain  --->
            (X) Arm ARM 2019.03
            ( ) Linaro ARM 2018.05
            ( ) Custom toolchain
                *** Toolchains from my-br2-ext-tree: ***
            ( ) My custom ARM toolchain
                *** Toolchains from another-br2-ext-tree: ***
            ( ) Another custom ARM toolchain
            ( ) A third custom ARM toolchain

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>