pmb: flasher: frontend: don't fail if kernel config cannot be found (MR 2184)

When using a kernel from Alpine the kernel configuration cannot be
found in pmaports. We cannot check the kernel config for missing
options in that case, but that's no reason to break the flasher
entirely.

test/test_kconfig_check.py

@@ -21,6 +21,10 @@ def args(tmpdir, request):
 
 
 def test_kconfig_check(args):
+    # non-existing package
+    assert pmb.parse.kconfig.check(args, "non-existing-kernel-package",
+                                   must_exist=False) is None
+
     # basic checks, from easiers to hard-ish
     dir = f"{pmb_test.const.testdata}/kconfig_check/"
     assert not pmb.parse.kconfig.check_file(dir +