fix(cm): hide `cm_init_context_by_index` from BL1

BL1 requires the context management library but does not use or
implement `cm_init_context_by_index`. This change ensures that is not
compiled into BL1, as linking with LTO enabled causes an undefined
reference for this function.

Change-Id: I4a4602843bd75bc4f47b3e0c4c5a6efce1514ef6
Signed-off-by: Chris Kay <chris.kay@arm.com>