linux-gbmc: Add task for userspace programs that need headers

This makes it possible for programs to depend on the userspace headers
from the kernel by adding:

```
CFLAGS:append = " -I${STAGING_KERNEL_BUILDDIR}/include"
do_configure[depends] += "virtual/kernel:do_shared_headers"
DEPENDS += "virtual/kernel"
```

Change-Id: Icc3278618d0c03d65cd7b59cea7a7e4092d3b52a
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/recipes-kernel/linux/linux-gbmc.inc b/recipes-kernel/linux/linux-gbmc.inc
index b6de687..d4bb4b8 100644
--- a/recipes-kernel/linux/linux-gbmc.inc
+++ b/recipes-kernel/linux/linux-gbmc.inc
@@ -35,3 +35,14 @@
   file://0003-mmc-aspeed-Adjust-delay-taps-calculation-method.patch \
   file://0004-mmc-aspeed-fix-sdhci-software-reset-can-t-be-cleared.patch \
 "
+
+do_compile_headers() {
+  oe_runmake -C ${S} O=${B} headers
+}
+
+do_shared_headers() {
+  cp -r ${B}/usr/include/ ${STAGING_KERNEL_BUILDDIR}
+}
+
+addtask do_compile_headers after do_configure
+addtask do_shared_headers after do_shared_workdir do_compile_headers