toolchain/external/predefined: add choice to select 'AT_LEAST_XXX' header version

Select the appropriate BR2_TOOLCHAIN_HEADERS_AT_LEAST_XXX options for the
external, pre-defined toolchains.

Also annotate those toolchain with older-than-3.0 headers.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>