linux: add BR2_LINUX_KERNEL_NEEDS_HOST_LIBELF

linux: add BR2_LINUX_KERNEL_NEEDS_HOST_LIBELF

Some Linux kernel configuration options (such as CONFIG_UNWINDER_ORC)
require building a host program that needs libelf.

Users who have libelf installed on their system won't see a problem,
but users who don't have libelf installed will get a build
failure. Therefore, this commit adds an option that allows a user to
indicate that his Linux kernel configuration requires libelf. When
this option is enabled, we add host-elfutils to the dependencies of
the linux package (host-elfutils provides the libelf library).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>