config: reorder the kernels sub-menu

Re-organise the sub-menu so that:
- the kernels list comes first,
- followed by kernels generic options
- followed by kernels specific options

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Linux kernel options

config KERNEL_linux

    select KERNEL_SUPPORTS_SHARED_LIBS

    help

      Build a toolchain targeting systems running Linux as a kernel.