unix: use pkg-config in sconscript file, add shell.nix for nix-shell

SConscript.unix

@@ -100,8 +100,6 @@ SOURCE_MOD += [
 ]
 if ARGUMENTS.get('TREZOR_EMULATOR_NOUI', 0):
     CPPDEFINES_MOD += ['TREZOR_EMULATOR_NOUI']
-else:
-    LIBS_MOD += ['SDL2', 'SDL2_image']
 
 # modtrezorutils
 SOURCE_MOD += [
@@ -273,6 +271,9 @@ env.Replace(
     ] + CPPDEFINES_MOD,
     ASPPFLAGS='$CFLAGS $CCFLAGS', )
 
+if not ARGUMENTS.get('TREZOR_EMULATOR_NOUI', 0):
+    env.ParseConfig('pkg-config --cflags --libs sdl2 SDL2_image')
+
 env.Replace(
     PYTHON='python',
     MAKEQSTRDATA='$PYTHON vendor/micropython/py/makeqstrdata.py',

shell.nix

@@ -0,0 +1,9 @@
+with import <nixpkgs> {};
+
+let
+  myPython = python36.withPackages(p: [p.trezor]);
+in
+  stdenv.mkDerivation {
+    name = "trezor-core-dev";
+    buildInputs = [ myPython protobuf scons gnumake gcc pkgconfig SDL2 SDL2_image ];
+  }