greybus: makefile: use POSIX functions for kernel cmp

[[, == and echo -e are bash/zsh-ism and not POSIX, so when using a POSIX
shell the kernel_cmp can issue some warnings and not work properly.

Use only POSIX operators for kernel version compare.
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/Makefile

@@ -61,7 +61,7 @@ ifneq ($(KERNELRELEASE),)
 # This function returns the argument version if current kernel version is minor
 # than the passed version, return 1 if equal or the current kernel version if it
 # is greater than argument version.
-kvers_cmp=$(shell [[ "$(KERNELVERSION)" == "$(1)" ]] && echo 1 || echo -e "$(1)\n$(KERNELVERSION)" | sort -V | tail -1)
+kvers_cmp=$(shell [ "$(KERNELVERSION)" = "$(1)" ] && echo 1 || printf "$(1)\n$(KERNELVERSION)" | sort -V | tail -1)
 
 ifneq ($(call kvers_cmp,"3.19.0"),3.19.0)
     CONFIG_OPTIONS_ENABLE += LEDS_CLASS_FLASH