makefile: Add help target
makefile: Add help target

This just adds some basic output as a reminder of what targets
do what.

BUG=none
BRANCH=none
TEST=make help shows useful output

Change-Id: Ic46f153ce8ed0b39294644a01eba8e06f2abe1c6
Signed-off-by: Martin Roth <79230511ccf79da3f7e9e82dbcc9c1ec2f734fa0@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/403497
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec