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>