package/linux-headers: select appropriate 'AT_LEAST_XXX' header version

Select the appropriate BR2_TOOLCHAIN_HEADERS_AT_LEAST_XXX options for the
internal toolchain backend.

For the manualy-specified version, we bring up the choice that sets the
_AT_LEAST_XXX symbols.

[Thomas: rename symbols from BR2_PACKAGE_HOST_LINUX_HEADERS_<version>
to BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_<version> to clearly indicate
that they are used to specify the version of custom 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>
Cc: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>