elogtool: add "clear" command

Adds "clear" command to cbfsutil/elogtool tool.
"clear" clears the RW_ELOG region by using either:
 * flashrom if no file is provided
 * or using file write if an input file is provided.

The region is filled with ELOG_TYPE_EOL. And a
ELOG_TYPE_LOG_CLEAR event is inserted.

Additionally, it does a minor cleanup to command "list", like:
 * use buffer_end()
 * add "list" to the cmds struct
 * and make elog_read() very similar to elog_write()

Usage:
$ elogtool clear

BUG=b:172210863
TEST=elogtool clear && elogtool list
     elogtool clear -f invalid.raw
     elogtool clear -f valid.raw

Change-Id: Ia28a6eb34c82103ab078a0841b022e2e5e430585
Signed-off-by: Ricardo Quesada <ricardoq@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56883
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>

util/cbfstool/eventlog.h

@@ -3,8 +3,13 @@
 #ifndef EVENTLOG_H_
 #define EVENTLOG_H_
 
+#include <stdint.h>
+
 struct event_header;
+struct buffer;
 
 void eventlog_print_event(const struct event_header *event, int count);
+int eventlog_init_event(const struct buffer *buf, uint8_t type,
+			const void *data, int data_size);
 
 #endif // EVENTLOG_H_