hikey: Add sleep to prevent umount failure

Sometimes it happens that the umount fails saying that a device/resource
is busy when trying umount the device after the filesystem has been
created. We don't know how and why this happens, but in the meantime
while waiting for a proper solution we have added a sleep that seems to
solve the problem.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

hikey.mk

@@ -316,6 +316,11 @@ boot-img: linux update_rootfs
 	sudo cp $(LINUX_PATH)/arch/arm64/boot/Image $(LINUX_PATH)/arch/arm64/boot/dts/hi6220-hikey.dtb .tmpbootimg/
 	sudo cp $(GEN_ROOTFS_PATH)/filesystem.cpio.gz .tmpbootimg/initrd.img
 	sudo cp $(EDK2_PATH)/Build/HiKey/$(EDK2_BUILD)_GCC49/AARCH64/AndroidFastbootApp.efi .tmpbootimg/fastboot.efi
+	# We cannot figure out why we need the sleep here, but from time to time
+	# we can see that we get "device/resource busy" when trying to unmount
+	# .tmpbootimg below. A short sleep seems to solve the problem and has to
+	# be here until we figure out why this happens.
+	sleep 3
 	sudo umount .tmpbootimg
 	sudo rm -rf .tmpbootimg