The 'impl' variable is guarded by the symbol DEBUG, but used in an INFO
level print statement. INFO is defined based on LOG_LEVEL. Hence, builds
would fail when
 - DEBUG=0 && LOG_LEVEL>=LOG_LEVEL_INFO with a variable used but not defined
 - DEBUG=1 && LOG_LEVEL<LOG_LEVEL_INFO with a variable defined but not used

Fixing this by guarding impl with the same condition that guards INFO.

Fixes ARM-software/tf-issues#490
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>