[zephyr] Fix zephyr compilation flag retrieval (#4422)
[zephyr] Fix zephyr compilation flag retrieval (#4422)

We have a mechanism to obtain Zephyr compilation flags,
so that we can pass some of them (like include paths etc.)
to CHIP build system. The mechanism leverages
Zephyr-supplied CMake functions which were supposed to
return lists of flags. However, in recent Zephyr revisions,
they changed behavior of the functions and the build fails.

Switch to using functions which return strings of flags
which seem to have more stable API.
nestlabs/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip