pmb: Make RunOutputTypeDefault and RunOutputTypePopen enums

This allows us to get rid of some of the validation in sanity_checks()
as mypy handles this validation at "build time", and any typos in the
enum instantiation would be a runtime error rather than a silent
failure.

Additionally, it allows us to encode some of the behaviour of the
different output types into the type definition itself by using methods.

Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2642
(cherry picked from commit 7d2f055bcb)

pmb: Fix PmbArgs containing a string for output

While the annotations were changed to suggest that the output property
of PmbArgs contains a RunOutputType, at runtime it actually contained a
string because the argument parsing code hadn't been adapted to create a
RunOutputType. Fix this, and also change it to RunOutputTypeDefault as
while at it as that's more accurate.

Fixes 7d2f055bcb
Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2644

pmb/flasher/frontend.py

@@ -17,6 +17,7 @@ import pmb.helpers.frontend
 import pmb.helpers.mount
 import pmb.parse.kconfig
 from pmb.core import Chroot, ChrootType
+from pmb.types import RunOutputTypeDefault
 
 
 def kernel(
@@ -192,7 +193,9 @@ def flash_lk2nd(
         pmb.flasher.init(deviceinfo.codename, method)
         logging.info("(native) checking current fastboot product")
         output = pmb.chroot.root(
-            ["fastboot", "getvar", "product"], output="interactive", output_return=True
+            ["fastboot", "getvar", "product"],
+            output=RunOutputTypeDefault.INTERACTIVE,
+            output_return=True,
         )
         # Variable "product" is e.g. "LK2ND_MSM8974" or "lk2nd-msm8226" depending
         # on the lk2nd version.