devtool workflow seems to fail if applied to the example external kernel module.

How to reproduce (see the tested commits in the attached configuration that follows):

1. add meta-skeleton to bblayers.conf
2. 'bitbake hello-mod' succeeds
3. 'devtool modify hello-mod' succeeds (it warns no source was unpacked to S, but then notes local source files have been added to srctree, which seems fine)
4. 'devtool build hello-mod' fails with:


Build Configuration:
BB_VERSION        = "1.32.0"
BUILD_SYS         = "x86_64-linux"
NATIVELSBSTRING   = "universal"
TARGET_SYS        = "i586-poky-linux"
MACHINE           = "qemux86"
DISTRO            = "poky"
DISTRO_VERSION    = "2.2"
TUNE_FEATURES     = "m32 i586"
TARGET_FPU        = ""
meta              
meta-poky         
meta-yocto-bsp    
meta-skeleton     = "master:1655d55764eb0241b71217810500fcf1d4519a1c"
workspace         = "<unknown>:<unknown>"

NOTE: Executing SetScene Tasks
NOTE: Executing RunQueue Tasks
hello-mod-0.1-r0 do_compile: NOTE: hello-mod: compiling from external source tree /home/gizero/work/upstreaming/build/workspace/sources/hello-mod
ERROR: hello-mod-0.1-r0 do_compile: oe_runmake failed
ERROR: hello-mod-0.1-r0 do_compile: Function failed: do_compile (log file is located at /home/gizero/work/upstreaming/build/tmp/work/qemux86-poky-linux/hello-mod/0.1-r0/temp/log.do_compile.630)
ERROR: Logfile of failure stored in: /home/gizero/work/upstreaming/build/tmp/work/qemux86-poky-linux/hello-mod/0.1-r0/temp/log.do_compile.630
Log data follows:
| DEBUG: Executing python function externalsrc_compile_prefunc
| NOTE: hello-mod: compiling from external source tree /home/gizero/work/upstreaming/build/workspace/sources/hello-mod
| DEBUG: Python function externalsrc_compile_prefunc finished
| DEBUG: Executing shell function do_compile
| NOTE: make -j 4 KERNEL_SRC=/home/gizero/work/upstreaming/build/tmp/work-shared/qemux86/kernel-source KERNEL_PATH=/home/gizero/work/upstreaming/build/tmp/work-shared/qemux86/kernel-source KERNEL_VERSION=4.8.6-yo
cto-standard CC=i586-poky-linux-gcc  -fuse-ld=bfd LD=i586-poky-linux-ld.bfd  AR=i586-poky-linux-ar  O=/home/gizero/work/upstreaming/build/tmp/work-shared/qemux86/kernel-build-artifacts KBUILD_EXTRA_SYMBOLS=
| ERROR: oe_runmake failed
| make: *** No targets specified and no makefile found.  Stop.
| ERROR: Function failed: do_compile (log file is located at /home/gizero/work/upstreaming/build/tmp/work/qemux86-poky-linux/hello-mod/0.1-r0/temp/log.do_compile.630)
ERROR: Task (/home/gizero/work/upstreaming/poky/meta-skeleton/recipes-kernel/hello-mod/hello-mod_0.1.bb:do_compile) failed with exit code '1'
NOTE: Tasks Summary: Attempted 436 tasks of which 435 didn't need to be rerun and 1 failed.

Summary: 1 task failed:
  /home/gizero/work/upstreaming/poky/meta-skeleton/recipes-kernel/hello-mod/hello-mod_0.1.bb:do_compile
Summary: There were 2 ERROR messages shown, returning a non-zero exit code.