kernel-fitimage.bbclass: do_assemble_fitimage(): cd to $B

Prior to assembling the fitimage, ensure that $B is the cwd due to
bitbake commit 67a7b8b021badc17d8fdf447c250e79d291e75f7 "build: don't
use $B as the default cwd for functions".

Without this change, do_assemble_fitimage() fails like:

Log data follows:
| DEBUG: Executing shell function do_assemble_fitimage
| arm-ka-linux-gnueabi-objcopy: 'vmlinux': No such file
| WARNING: exit code 1 from a shell command.
| ERROR: Function failed: do_assemble_fitimage

(From OE-Core rev: 42d50e8f5f3a98e50a0f50473ebc83dc6347b634)

Change-Id: I810ffe67f5edad12064cfab17bd72f2b8e1b5191
Signed-off-by: Andrew Bradford <andrew.bradford@kodakalaris.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Rick Altherr <raltherr@google.com>
diff --git a/import-layers/yocto-poky/meta/classes/kernel-fitimage.bbclass b/import-layers/yocto-poky/meta/classes/kernel-fitimage.bbclass
index 7e6ea98..526df9b 100644
--- a/import-layers/yocto-poky/meta/classes/kernel-fitimage.bbclass
+++ b/import-layers/yocto-poky/meta/classes/kernel-fitimage.bbclass
@@ -156,6 +156,7 @@
 }
 
 do_assemble_fitimage() {
+	cd ${B}
 	if echo ${KERNEL_IMAGETYPES} | grep -wq "fitImage"; then
 		kernelcount=1
 		dtbcount=""