cmake: correct debugger feature flag

The SCP_ENABLE_DEBUGGER_INIT variable is used to provide a default value
to the SCP_ENABLE_DEBUGGER option in the case that it is availiable. However,
in Makefile.cmake and a selection of products, SCP_ENABLE_DEBUGGER_INIT
is treated as the user option itself rather than SCP_ENABLE_DEBUGGER.
This patch retifies these files to correctly refer to SCP_ENABLE_DEBUGGER.

Signed-off-by: Brett Warren <brett.warren@arm.com>
Change-Id: Iafcc93d1e54a6342774165e900c03f85102d2a00