build warnings when updating to master from 2.2.0

[pabigot]

Somewhere between 2.2.0 and master a change was made that introduced an API change that is not detected when initiating a non-clean build in an existing directory.

Steps to reproduce:

cd ${ZEPHYR_ROOT}/samples/hello_world
git checkout v2.2.0
west update
rm -rf build && west build -b nrf52840_pca10056
git checkout fe3e12face # or upstream master
west update
west build

Last step produces diagnostics suggesting something should have been rebuilt that wasn't:

tirzah[163]$ west build
[0/1] Re-running CMake...
-- Application: /mnt/nordic/zp/zephyr/samples/hello_world
-- Zephyr version: 2.2.99
-- Board: nrf52840_pca10056
-- Found west: /home/pab/.local/bin/west (found suitable version "0.7.1", minimum required is "0.6.0")
-- Found toolchain: zephyr (/usr/local/zephyr-sdk)
-- Loading /mnt/nordic/zp/zephyr/boards/arm/nrf52840_pca10056/nrf52840_pca10056.dts as base
Devicetree header saved to '/mnt/nordic/zp/zephyr/samples/hello_world/build/zephyr/include/generated/devicetree_unfixed.h'
Parsing /mnt/nordic/zp/zephyr/Kconfig
Loaded configuration '/mnt/nordic/zp/zephyr/samples/hello_world/build/zephyr/.config'
No change to configuration in '/mnt/nordic/zp/zephyr/samples/hello_world/build/zephyr/.config'
No change to Kconfig header in '/mnt/nordic/zp/zephyr/samples/hello_world/build/zephyr/include/generated/autoconf.h'
-- Cache files will be written to: /home/pab/.cache/zephyr
-- Configuring done
-- Generating done
-- Build files have been written to: /mnt/nordic/zp/zephyr/samples/hello_world/build
[2/90] Building C object zephyr/CMakeFiles/offsets.dir/arch/arm/core/offsets/offsets.c.obj
In file included from /mnt/nordic/zp/zephyr/arch/arm/core/offsets/offsets_aarch32.c:25,
                 from /mnt/nordic/zp/zephyr/arch/arm/core/offsets/offsets.c:12:
../../../include/kernel.h:937:15: warning: 'k_thread_join' declared 'static' but never defined [-Wunused-function]
  937 | __syscall int k_thread_join(struct k_thread *thread, s32_t timeout);
      |               ^~~~~~~~~~~~~
[8/90] Building C object zephyr/arch/arch/arm/core/aarch32/cortex_m/CMakeFiles/arch__arm__core__aarch32__cortex_m.dir/fault.c.obj
In file included from /mnt/nordic/zp/zephyr/arch/arm/core/aarch32/cortex_m/fault.c:14:
../../../include/kernel.h:937:15: warning: 'k_thread_join' declared 'static' but never defined [-Wunused-function]
  937 | __syscall int k_thread_join(struct k_thread *thread, s32_t timeout);
      |               ^~~~~~~~~~~~~
[9/90] Building C object zephyr/arch/arch/arm/core/aarch32/cortex_m/CMakeFiles/arch__arm__core__aarch32__cortex_m.dir/scb.c.obj
In file included from /mnt/nordic/zp/zephyr/arch/arm/core/aarch32/cortex_m/scb.c:17:
../../../include/kernel.h:937:15: warning: 'k_thread_join' declared 'static' but never defined [-Wunused-function]
  937 | __syscall int k_thread_join(struct k_thread *thread, s32_t timeout);