platform/target: Implement weak functions for boot device apis

Boot device related apis are used only on targets starting apq8084, msm8994
etc. We implement weak functions to enable successful compilation for
targets which doesnt use this function.

CRs-Fixed: 612217
Change-Id: I84f1a28c511cecceb0f7b8914798a3b136cb57b4
3 files changed