build: unify verbosity handling

This change introduces a few helper variables for dealing with verbose
and silent build modes: `silent`, `verbose`, `q` and `s`.

The `silent` and `verbose` variables are boolean values determining
whether the build system has been configured to run silently or
verbosely respectively (i.e. with `--silent` or `V=1`).

These two modes cannot be used together - if `silent` is truthy then
`verbose` is always falsy. As such:

    make --silent V=1

... results in a silent build.

In addition to these boolean variables, we also introduce two new
variables - `s` and `q` - for use in rule recipes to conditionally
suppress the output of commands.

When building silently, `s` expands to a value which disables the
command that follows, and `q` expands to a value which supppresses
echoing of the command:

    $(s)echo 'This command is neither echoed nor executed'
    $(q)echo 'This command is executed but not echoed'

When building verbosely, `s` expands to a value which disables the
command that follows, and `q` expands to nothing:

    $(s)echo 'This command is neither echoed nor executed'
    $(q)echo 'This command is executed and echoed'

In all other cases, both `s` and `q` expand to a value which suppresses
echoing of the command that follows:

    $(s)echo 'This command is executed but not echoed'
    $(q)echo 'This command is executed but not echoed'

The `s` variable is predominantly useful for `echo` commands, where you
always want to suppress echoing of the command itself, whilst `q` is
more useful for all other commands.

Change-Id: I8d8ff6ed714d3cb401946c52955887ed7dca602b
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/plat/arm/board/arm_fpga/platform.mk b/plat/arm/board/arm_fpga/platform.mk
index 520a73a..cb78e35 100644
--- a/plat/arm/board/arm_fpga/platform.mk
+++ b/plat/arm/board/arm_fpga/platform.mk
@@ -134,7 +134,7 @@
 endif
 
 bl31.axf: bl31 dtbs ${BUILD_PLAT}/rom_trampoline.o ${BUILD_PLAT}/kernel_trampoline.o ${BUILD_PLAT}/build_axf.ld
-	$(ECHO) "  LD      $@"
-	$(Q)$($(ARCH)-ld) -T ${BUILD_PLAT}/build_axf.ld -L ${BUILD_PLAT} $(TF_LDFLAGS) $(PLAT_LDFLAGS) -s -n -o ${BUILD_PLAT}/bl31.axf
+	$(s)echo "  LD      $@"
+	$(q)$($(ARCH)-ld) -T ${BUILD_PLAT}/build_axf.ld -L ${BUILD_PLAT} $(TF_LDFLAGS) $(PLAT_LDFLAGS) -s -n -o ${BUILD_PLAT}/bl31.axf
 
 all: bl31.axf