init: moving get_block_device_symlinks() to public API

This is required for init to get by-name symlinks in the first stage
mount with AVB.

Bug: 37552224
Test: first stage mount /vendor with vboot 2.0 (AVB)
Change-Id: Ib067753f651ecea4d4d26215da9294f1c607d95e
Merged-In: Ia0f290542eb1cffce5ae876dfedb453dde960253
2 files changed