Create empty /linkerconfig/ld.config.txt in ramdisk.

This silences warnings such as:

linker: Warning: failed to find generated linker configuration from "/linkerconfig/ld.config.txt"
WARNING: linker: Warning: failed to find generated linker configuration from "/linkerconfig/ld.config.txt"

that are printed at the start of every command in the first
stage console.

Change-Id: I3d820e4960a4dbca34369223aed83589534c2eb6