Makefile: Support totally quiet output with -s

"-s" is a command line option to the make tool, to suppress normal output,
something to the effect of prepending every line with '@' in the Makefile.
However with our V={0|1} support, we now print the shortened command line
output in any case (even with V=1, in addition to the long line!).
Normally -s helps to not miss non-fatal warnings, which tend to scroll out
of the window easily.

Introduce a new Makefile variable ECHO, to control the shortened output.
We only set it in the (current default) V=0 case, and replace every
occurence of "@echo" with that variable.
When the user specifies "-s", we set ECHO to some magic string which
changes the output line into a comment, so the output is suppressed.

Beside suppressing every output for "-s", we also avoid the redundant
short output when compiling with V=1.

This changes the output to:
==========
$ make -s PLAT=.... bl31

Built build/.../release/bl31.bin

==========
$ make PLAT=.... bl31
...
  CC      lib/libc/strncmp.c
  CC      lib/libc/strnlen.c
...
==========
$ make V=1 PLAT=.... bl31
...
gcc -DDEBUG=0 .... -o build/.../release/libc/strncmp.o
gcc -DDEBUG=0 .... -o build/.../release/libc/strnlen.o
...
==========

Signed-off-by: Andre Przywara <andre.przywara@arm.com>