gen_relocate_app.py: make generated/linker_relocate.ld deterministic

Dictionaries are not ordered in Python 3.5 and before, so building twice
${ZEPHYR_BASE}/samples/application_development/code_relocation/
in a row could lead to a different sections order, different
build/zephyr/include/generated/linker_relocate.d and code_relocation.c
and different binaries.

Fix with a minor change to three "for" loops in the output functions:
make them iterate on sorted(list of sections) instead of the raw and
randomly ordered dictionaries of sections.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>