net: lib: app: Convert CMakeLists.txt to avoid library
net: lib: app: Convert CMakeLists.txt to avoid library

Due to the fact that cmake doesn't allow to build empty "library"
(which would be the case when neither CONFIG_NET_APP_SERVER nor
CONFIG_NET_APP_CLIENT is defined).

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,explora26/zephyr,finikorg/zephyr,punitvara/zephyr,kraj/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr,punitvara/zephyr,finikorg/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,ldts/zephyr,kraj/zephyr,explora26/zephyr,explora26/zephyr,explora26/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,kraj/zephyr,punitvara/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,punitvara/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,ldts/zephyr