Provide hook for developer local PIO configuration (speeduino#1212)

.gitignore

@@ -26,4 +26,5 @@ reference/doxygen
 speeduino.ino.cpp
 test/output_export.cpp
 misra/.results
-~$*
+~$*
+local.ini

platformio.ini

@@ -25,12 +25,12 @@ test_ignore = test_table3d_native
 ;This environment is the same as the above, however compiles for 6 channels of fuel and 3 channels of ignition
 [env:megaatmega2560-6-3]
 extends = env:megaatmega2560
-build_flags = -DUSE_LIBDIVIDE -O3 -ffast-math -fshort-enums -funroll-loops -Wall -Wextra -std=c99 -DINJ_CHANNELS=6 -DIGN_CHANNELS=3
+build_flags = ${env:megaatmega2560.build_flags} -DINJ_CHANNELS=6 -DIGN_CHANNELS=3
 
 ;As the above, however compiles for 8 channels of fuel and only a single ignition channel
 [env:megaatmega2560-8-1]
 extends = env:megaatmega2560
-build_flags = -DUSE_LIBDIVIDE -O3 -ffast-math -fshort-enums -funroll-loops -Wall -Wextra -std=c99 -DINJ_CHANNELS=8 -DIGN_CHANNELS=1
+build_flags = ${env:megaatmega2560.build_flags} -DINJ_CHANNELS=8 -DIGN_CHANNELS=1
 
 [env:megaatmega2561]
 extends = env:megaatmega2560
@@ -150,6 +150,10 @@ monitor_speed = 115200
 [platformio]
 src_dir=speeduino
 default_envs = megaatmega2560
+; local.ini is ignored via .gitignore
+; This allows developers to make private/local PIO configuration changes without 
+; triggering Git changes in their dev tooling (E.g. VS Code)  
+extra_configs = local.ini
 ;The following lines are for testing / experimentation only. Comment the line above to try them out
 ;default_envs = black_F407VE
 ;default_envs = teensy35