examples: make examples/ optional

Most users don't need the standalone API examples. Distributions like SUSE
do not supply libgcc for cross-compiling and we cannot do without on ARMv8
for building examples/.

Make examples selectable via symbol CONFIG_EXAMPLES. It defaults to
yes on ARCH_QEMU to ensure that we compile the API as part of our
continuous integration.

Cc: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>