Create new module type 'snapshot_etc'

It requires new module type to generate 'prebuilt_etc' modules in the
snapshot, because of name conflict with existing modules. This change
defines new module type 'snapshot_etc' as prebuilt of 'prebuilt_etc', so
it can replace original module on certain conditions.

Bug: 192430376
Test: Tested with snapshot_etc of ld.config.recovery.txt module
Change-Id: Ia4d27ce2077a10591597d2614c7c29c23a245149