kconfig check: add --keep-going argument (MR 2384)

Abort the "pmbootstrap kconfig check" on the first error, unless the
--keep-going argument was passed. This makes it easier to go through the
failed kernels one by one until they are all fixed.

pmb/types.py

@@ -99,6 +99,7 @@ class PmbArgs(Namespace):
     kconfig_check_details: bool
     kernel: str
     keymap: str
+    keep_going: bool
     lines: int
     log: Path
     mirror_alpine: str