envkernel: Add build command to create an apk package from envkernel (!1747)

Provides a quick way to incrementally compile a kernel and push it to
device.

Example usage.

Compile the kernel:
$ cd /src/linux/
$ source /src/pmbootstrap/helpers/envkernel.sh
$ make tegra_postmarketos_defconfig
$ make -jX

Package kernel and flash to device:
$ pmbootstrap build --envkernel linux-samsung-p4wifi
$ pmbootstrap flasher flash_kernel

Modify kernel source then incremental compile, package, and flash:
$ make -jX
$ pmbootstrap build --envkernel linux-samsung-p4wifi
$ pmbootstrap flasher flash_kernel

pmb/parse/arguments.py

@@ -493,6 +493,9 @@ def arguments():
                        " you don't need to build and install the kernel. But it"
                        " is incompatible with how Alpine's abuild handles it.",
                        dest="ignore_depends")
+    build.add_argument("--envkernel", action="store_true",
+                       help="Create an apk package from the build output of"
+                       " a kernel compiled with envkernel.sh.")
     for action in [checksum, build, aportgen]:
         argument_packages = action.add_argument("packages", nargs="+")
         if argcomplete: