build(core): SCons "go fast" button

some pretty safe & reasonable defaults to speed up SCons:

- look at timestamps to figure out whether a file has changed
- but only if the timestamp is older than 10 secs (which should avoid any problems with 1s precision of mtime, as well as clock drifts etc)
- implicit cache is something to do with parsing C files for dependencies

see https://github.com/SCons/scons/wiki/GoFastButton

core/SConstruct

@@ -1,4 +1,7 @@
 # pylint: disable=E0602
+Decider('content-timestamp')
+SetOption('max_drift', 10)
+SetOption('implicit_cache', 1)
 
 SConscript('SConscript.boardloader', variant_dir='build/boardloader', duplicate=False)
 SConscript('SConscript.bootloader', variant_dir='build/bootloader', duplicate=False)