gen_relocate_app: Create files from scratch, do not append

The problem with append is that when doing incremental (non-clean)
build, the content of these files are appended each time to the previous
leftover content.

Example:

 # west build -p -b qemu_cortex_m3 \
 	samples/application_development/code_relocation/

 #   // disable for example CONFIG_XIP

 # west build -b qemu_cortex_m3 \
        samples/application_development/code_relocation/

at this point the content of the generated files (i.e.
linker_relocate.ld) is wrong.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>