jpeg core example, migrate to flake

.gitmodules

@@ -2,6 +2,9 @@
 	path = kyupy
 	url = https://git.vlab.cse.kyutech.ac.jp/stefan/kyupy.git
 	branch = devel
-[submodule "circuits/picorv32"]
+[submodule "picorv32"]
 	path = picorv32
-	url = git@github.com:s-holst/picorv32.git
+	url = git@github.com:YosysHQ/picorv32.git
+[submodule "core_jpeg"]
+	path = core_jpeg
+	url = https://github.com/ultraembedded/core_jpeg.git