pmb.config.init.boot_size: set to 256 MiB (MR 2037)

Increase the boot partition size from 128 MiB to 256 MiB, as we are
already close to reaching the current limit. By having twice the size,
it should be possible to have atomic replacements of all initramfs
related files.

pmb/config/__init__.py

@@ -105,7 +105,7 @@ defaults = {
     "ui_extras": False,
     "user": "user",
     "work": os.path.expanduser("~") + "/.local/var/pmbootstrap",
-    "boot_size": "128",
+    "boot_size": "256",
     "extra_space": "0",
     "sudo_timer": False
 }