This commit allows the `QEMU_KERNEL_OPTION` variable, which can be
overridden by the `board.cmake`, to contain references to the variables
that are not available at the time of `board.cmake` inclusion, by
expanding its escpaed variable references in `cmake/emu/qemu.cmake`
which is included nearby the end of the root `CMakeLists.txt`.
With this change, the `board.cmake` can escape variable references as
follows and allow them to be expanded later:
set(QEMU_KERNEL_OPTION "-device;loader,file=\$<TARGET_FILE:$\{...}>")
Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
19965f8173