pmb/build/kconfig.py: rename from menuconfig.py

Prepare to add more kernel config handling code to that file, that is
not related to running menuconfig.

Reviewed-by: Clayton Craft <clayton@craftyguy.net>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230312151325.1968-14-ollieparanoid@postmarketos.org%3E

pmb/build/__init__.py

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 from pmb.build.init import init, init_compiler
 from pmb.build.envkernel import package_kernel
-from pmb.build.menuconfig import menuconfig
+from pmb.build.kconfig import menuconfig
 from pmb.build.newapkbuild import newapkbuild
 from pmb.build.other import copy_to_buildpath, is_necessary, \
     index_repo