qemu: add support for host-qemu-system

qemu: add support for host-qemu-system

This commit adds support for building the system emulation mode in
host-qemu. To do so, it adds the BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE and
BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE sub-options, making sure that the
latter is selected if the former is not enabled. This ensures that at
least one is enabled *and* that existing configurations continue to
build the user-land emulation (which was the only one we supported until
now).

The list of architectures supported by the system emulation mode is the
same as the one for the user-space emulation mode (as far as the
existing list is concerned), so we simply drop the comment about this
dependency list being related to the user-space emulation only.

Signed-off-by: Simon Maes <simonn.maes@gmail.com>
[Thomas:
 - Make sure either BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE or
   BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE get selected.
 - Unconditionally enable FDT support in host-qemu when in system
   mode (so the option BR2_PACKAGE_HOST_QEMU_FDT has been removed)
 - Remove the unneeded BR2_PACKAGE_HOST_QEMU_HAS_EMULS option.
 - Remove the SDL related option, we really on the system to provide
   SDL.
 - Remove the BR2_PACKAGE_HOST_QEMU_DEBUG and
   BR2_PACKAGE_HOST_QEMU_STRIP_BINARY options, since they are not really
   useful.
 - Remove HOST_QEMU_SITE and HOST_QEMU_SOURCE definitions, since they
   are automatically derived from QEMU_SITE and QEMU_SOURCE anyway.
 - Group things more logically in the .mk file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>