tools/lxterm/json: json file provide relative path, add json file directory to image names.

Allow sharing same json file between serial boot and Ethernet/SDCard/SATAboot:

boot.json:
{
	"Image":        "0x40000000",
	"rv32.dtb":     "0x40ef0000",
	"rootfs.cpio":  "0x41000000",
	"opensbi.bin":  "0x40f00000"
}

If boot.json and images are located in images directory, using lxterm --images=images/boot.json
will now work.

litex/tools/litex_term.py

@@ -229,7 +229,9 @@ class LiteXTerm:
             self.boot_address = kernel_address
         if json_images is not None:
             f = open(json_images, "r")
-            self.mem_regions.update(json.load(f))
+            json_dir = os.path.dirname(json_images)
+            for k, v in json.load(f).items():
+                self.mem_regions[os.path.join(json_dir, k)] = v
             self.boot_address = self.mem_regions[list(self.mem_regions.keys())[-1]]
             f.close()
         self.flash = flash