Decrease compile time with nanostack-libservice (#379)

nanostack-libservice involves two parts: common (e.g. libip4string) and
nanostack-specific (e.g. nvmHelper). For nanostack-specific, it needs to
link nanostack hal and will pull in 200+ nanostack files for compile.
This is unnecessary for common only application e.g. lwipstack. To fix this,
the following strategy is applied for nanostack-libservice cmake listfile:
1.  For compiling nanostack-specific, it doesn't rely on nanostack hal
    because its needed header file has placed underneath
    (mbed-client-libservice/platform).
2.  For application needing the nanostack-specific, the nanostack cmake
    listfile shall specify the link dependency.

connectivity/libraries/nanostack-libservice/CMakeLists.txt

@@ -22,8 +22,6 @@ target_sources(mbed-nanostack-libservice
         source/nvmHelper/ns_nvm_helper.c
 )
 
-target_link_libraries(mbed-nanostack-libservice PUBLIC mbed-nanostack-hal_mbed_cmsis_rtos)
-
 # The definition, source files and include directories below
 # are needed by mbed-trace which is part of the mbed-core CMake target
 target_compile_definitions(mbed-core-flags