aconfig: create one file at each create-storage command invocation

Based on the build system feedback, it is better to create storage file
individually per each aconfig command invocation in a makefile. Thus
updating the create-storage command to create one file at a time based
on passed in file selection.

Bug: b/321243743
Test: atest aconfig.test and manually invoke create-storage command
Change-Id: I53625c08e44d3ece4476835827c6b7d28ddd5a1b