staging.bbclass: print searched manifest when not found

staging.bbclass: print searched manifest when not found

The old warning was:
WARNING: lib32-wrlinux-image-glibc-small-1.0-r1 do_prepare_recipe_sysroot: Manifest /path/to/tmp/sstate-control/manifest-allarch-lib32-qemuwrapper-cross.populate_sysroot not found?

The message wasn't clear enough, it searched a few manifests, but only
reported the last one, which confused user.

Now the warning is:
WARNING: lib32-wrlinux-image-glibc-small-1.0-r1 do_rootfs: Manifest for lib32-qemuwrapper-cross not found, searched manifests:
/path/to/tmp/sstate-control/manifest-qemux86_64-lib32-qemuwrapper-cross.populate_sysroot
/path/to/tmp/sstate-control/manifest-core2-64-lib32-qemuwrapper-cross.populate_sysroot
/path/to/tmp/sstate-control/manifest-x86_64-lib32-qemuwrapper-cross.populate_sysroot
/path/to/tmp/sstate-control/manifest-allarch-lib32-qemuwrapper-cross.populate_sysroot

Whick makes debug easier.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>