trusty: Support directories as inputs to replace_ramdisk_modules.py

Add support for passing in a directory instead of an image file
to --kernel-ramdisk so the script can be used with the output
of `make modules_install` from a kernel build.

Bug: 364748249
Test: Used with https://r.android.com/3256316
Change-Id: Ica881c48a94bdc0ec990d521f19df8833e75c836