runqemu: improve finding of rootfs, kernel and dtb

runqemu: improve finding of rootfs, kernel and dtb

* Search rootfs in the following order:
  - IMAGE_NAME*.FSTYPE
  - IMAGE_LINK_NAME*.FSTYPE

* Search kernel in the following order:
  - QB_DEFAULT_KERNEL
  - KERNEL_IMAGETYPE
  - KERNEL_IMAGETYPE*

* Search dtb in the following order:
   - QB_DTB
   - QB_DTB*
   - *.dtb

* Fix DTB, it should only work with "-kernel" option.

[YOCTO #10265]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>