cmake: bintools abstraction, support <command>_flag_final property

This command add support for `<command>_flag_final` to all bintools
commands.

This allows users that has special use-cases to use a CMake script
for bintool command execution, and thereby have full control of command
invocation and argument processing.

Example of how to specify the property for such use:
Calling a custom script for elfconvert command:
set_property(TARGET bintools
             PROPERTY elfconvert_command ${CMAKE_COMMAND})
set_property(TARGET bintools
             PROPERTY elfconvert_flag "")
set_property(TARGET bintools
             PROPERTY elfconvert_flag_final     -P elfconvert.cmake)
set_property(TARGET bintools
             PROPERTY elfconvert_flag_strip_all "-DSTRIP_ALL=True")
set_property(TARGET bintools
             PROPERTY elfconvert_flag_infile    "-DINFILE=")
set_property(TARGET bintools
             PROPERTY elfconvert_flag_outfile   "-DOUT_FILE=")

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>