boards: qemu_x86_tiny: enabled for general demand paging testing

This enables qemu_x86_tiny to be used for more general demand
paging testing where non-pinned code and data is not available
in physical memory at boot. This adds a custom linker script to
qemu_x86_tiny for pinning code and data. In the process, a new
kconfig CONFIG_BOARD_QEMU_X86_TINY has to be introduced to
distinguish from other qemu_x86* boards. This linker script
is based on the generic x86-32 one in
include/arch/x86/ia32/linker.ld, with additions to
put symbols into boot and pinned sections.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>