kernel.bbclass: explicitly set workdir in do_bundle_initramfs

bitbake rev 67a7b8b02 "build: don't use $B as the default cwd for
functions" (included in current bitbake master) breaks the assumption
that do_bundle_initramfs runs inside the build directory.

This causes kernel_do_compile() as called from within
do_bundle_initramfs() to fail, as the former is not being executed
from the correct directory anymore. (Note that kernel_do_compile()
as called from bitbake directly doesn't suffer from that problem,
as it inherits the workdir from base_do_compile() in that case.)

Set workdir explicitly.

(From OE-Core rev: 4455da22a151c2ac006af63cbd39779b21b12580)

Signed-off-by: André Draszik <git@andred.net>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/kernel.bbclass

@@ -246,6 +246,7 @@ do_bundle_initramfs () {
 		done
 	fi
 }
+do_bundle_initramfs[dirs] = "${B}"
 
 python do_devshell_prepend () {
     os.environ["LDFLAGS"] = ''