build(measured boot): rename measured boot makefile

With the removal of the generic functions measured_boot_init()/finish(),
measured_boot.mk becomes specific to the TCG event log backend. Change
its file name to event_log.mk.
Also, the Event Log driver is one of the backend of measured boot hence
created a separate folder for it under the measured_boot directory.

Alongside done some cosmetic changes (adding a comment and fixing
identation).

Change-Id: I4ce3300e6958728dc15ca5cced09eaa01510606c
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/plat/arm/common/arm_common.mk b/plat/arm/common/arm_common.mk
index f82392c..a20e258 100644
--- a/plat/arm/common/arm_common.mk
+++ b/plat/arm/common/arm_common.mk
@@ -406,7 +406,7 @@
 endif
 
 ifeq (${MEASURED_BOOT},1)
-    MEASURED_BOOT_MK := drivers/measured_boot/measured_boot.mk
+    MEASURED_BOOT_MK := drivers/measured_boot/event_log/event_log.mk
     $(info Including ${MEASURED_BOOT_MK})
     include ${MEASURED_BOOT_MK}
 endif