toolchain-external: apply kernel headers check to non-custom toolchains

Since the introduction of the kernel headers Config.in options, the
external toolchain logic had a check for custom external toolchains to
verify that the kernel headers version entered by the user matches the
one of the toolchain. However, this check was not made for non-custom
external toolchains (i.e the built-in profiles, such as Linaro,
CodeSourcery and al.), making the assumption that the Buildroot
developers will do the right selection.

However, it is quite nice when bumping external toolchains to have
this automatic kernel headers version check, to ensure we select the
appropriate kernel headers version.

Therefore, this commit makes the kernel headers version check
applicable to non-custom external toolchains.

Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>