cmake: CMake linker functions implemented

This commit introduces zephyr_linker CMake functions for creation of
linker scripts.

The linker functions supports the following features:
- Configuration of memory sections
- Configuration of memory groups
- Creation of output sections
- Configuration of input section the should go into output section and
  how those should be treated, such as sorting, keep, priority.
- Defining linker symbols
- Specifying Kernel VMA to support virtual linking on x86

Overview of functions and macros introduce with this commit:
- pow2round
- zephyr_linker
- zephyr_linker_memory
- zephyr_linker_memory_ifdef
- zephyr_linker_group
- zephyr_linker_section
- zephyr_linker_section_ifdef
- zephyr_iterable_section
- zephyr_linker_section_obj_level
- zephyr_linker_section_configure
- zephyr_linker_symbol
- zephyr_linker_property_append

Signed-off-by: Torsten Rasmussen <[email protected]>

Author: tejlmand