[cmake] fix platform-utils for cmake (#4112)

platform-utils has a dependency on the core ot library, which was
causing the build to fail. I have resolved this by making
platform-utils an 'object library', which means the object files
will always be used by the linker, preventing the 'smart linker'
errors that occur with ld and cyclic dependencies.

I also provided a static (.a) implementation of the library in case
any non-cmake project wanted to consume the results (at which point
that project can deal with the dependency as it wishes).

So now it is up to the 'platform layer' to include the 'platform utils'
objects inside its own library if it requires them. I have implemented
this for the current cmake example platforms.

examples/apps/cli/CMakeLists.txt

@@ -48,7 +48,6 @@ target_link_libraries(ot-cli-ftd
     openthread-cli-ftd
     ${OT_PLATFORM_LIB}
     openthread-ftd
-    openthread-platform-utils
     ${OT_PLATFORM_LIB}
     mbedcrypto
 )
@@ -57,7 +56,6 @@ target_link_libraries(ot-cli-mtd
     openthread-cli-mtd
     ${OT_PLATFORM_LIB}
     openthread-mtd
-    openthread-platform-utils
     ${OT_PLATFORM_LIB}
     mbedcrypto
 )

examples/apps/ncp/CMakeLists.txt

@@ -53,7 +53,6 @@ target_link_libraries(ot-ncp-ftd
     openthread-ncp-ftd
     ${OT_PLATFORM_LIB}
     openthread-ftd
-    openthread-platform-utils
     ${OT_PLATFORM_LIB}
     mbedcrypto
 )
@@ -62,7 +61,6 @@ target_link_libraries(ot-ncp-mtd
     openthread-ncp-mtd
     ${OT_PLATFORM_LIB}
     openthread-mtd
-    openthread-platform-utils
     ${OT_PLATFORM_LIB}
     mbedcrypto
 )
@@ -71,6 +69,5 @@ target_link_libraries(ot-rcp
     openthread-rcp
     ${OT_PLATFORM_LIB}
     openthread-radio
-    openthread-platform-utils
     ${OT_PLATFORM_LIB}
 )