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

Select the appropriate BR2_TOOLCHAIN_HEADERS_AT_LEAST_XXX options for the
external, custom toolchain backend.

We try to be conservative here, and default to kernel headers 2.6.x.

[Thomas: remove duplicated depends on BR2_TOOLCHAIN_EXTERNAL_CUSTOM,
since the choice is already inside a if BR2_TOOLCHAIN_EXTERNAL_CUSTOM
... endif block.]

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>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>