cmake: check ZEPHYR_SDK_INSTALL_DIR is set

Check for ZEPHYR_SDK_INSTALL_DIR being invalid instead of
checking for SDK_VERSION being not defined. This change
relates to commit bb09c458c18f ("cmake: Prevent infinite
recursion").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
1 file changed