cmake: Introduce toolchain_ld_link_elf to abstract linker invocation

Final step of linker abstraction:
 * Abstract zephyr_lnk by including it in toolchain_ld_link_elf.
 * Abstract relevant uses of target_link_libraries.
 * Introduce toolchain_ld_force_undefined_symbols.

No functional change expected.

This is motivated by the wish to abstract Zephyr's usage of toolchains,
permitting non-intrusive porting to other (commercial) toolchains.

Signed-off-by: Mark Ruvald Pedersen <mped@oticon.com>
2 files changed