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>
7 files changed