core: enable firmware build without SDL2 installed

SConstruct includes SConscript.unix even for the firmware builds,
which resuls in error if SDL2 is not installed.
This commit workarounds the issue.

Is there a better way how to NOT include SConscript.unix via
SConstruct for firmware only builds?

core/SConscript.unix

@@ -349,7 +349,10 @@ env.Replace(
     ] + CPPDEFINES_MOD,
     ASPPFLAGS='$CFLAGS $CCFLAGS', )
 
-env.ParseConfig('pkg-config --cflags --libs sdl2 SDL2_image')
+try:
+    env.ParseConfig('pkg-config --cflags --libs sdl2 SDL2_image')
+except OSError:
+    print("SDL2 not installed, Emulator build is not possible")
 
 env.Replace(
     PYTHON='python',