* Helix/pico use post_rules.mk insted of local_features.mk
Tested with the following procedure.
```
git checkout 77a93fec79 (qmk/develop)
make clean
make helix/pico:all:check-md5
make helix/pico/sc:all:check-md5
git checkout helix_use_post_rules.mk
make clean
make helix/pico:all:check-md5
make helix/pico/sc:all:check-md5
```
* Helix/rev2 use post_rules.mk
Tested with the following procedure.
```
git checkout 77a93fec79 (qmk/develop)
make clean
make helix/pico:all:check-md5
make helix/pico/sc:all:check-md5
make helix/rev2:all:check-md5
make helix/rev2/sc:all:check-md5
git checkout helix_use_post_rules.mk
make clean
make helix/pico:all:check-md5
make helix/pico/sc:all:check-md5
make helix/rev2:all:check-md5
make helix/rev2/sc:all:check-md5
```
* Better handle LTO_ENABLE
Especially when calling from command line
* Replace LINK_TIME_OPTIMIZATION_ENABLE with LTO_ENABLE
* Remove long for LTO from show_options.mk
* initial commit for froggy 106 key mode
* add mode indicator on OLED
* use #pragma once instead of include guard
* remove unusable codes
* remove audio codes, because helix rev.2 has no audio feature
* use set_single_persistent_default_layer
* remove eeprom update check
