linux/ext: fix Xenomai unmet dependencies

linux/ext: fix Xenomai unmet dependencies

Properly propagate the Xenomai dependencies to the corresponding kernel
extension, to fix the following unmet dependencies:

    warning: (BR2_LINUX_KERNEL_EXT_XENOMAI) selects BR2_PACKAGE_XENOMAI
    which has unmet direct dependencies (BR2_PACKAGE_XENOMAI_ARCH_SUPPORTS
    && BR2_TOOLCHAIN_HAS_THREADS && !BR2_TOOLCHAIN_USES_MUSL)

While at it, move the comment lower, after the path option, so that the
path option is properly indented in the menuconfig.

Add markers to separate each extension.

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