event: Add basic support for events

Add a way to create and dispatch events without needing to allocate
memory. Also add a way to 'spy' on events, thus allowing 'hooks' to be
created.

Use a linker list for static events, which we can use to replace functions
like arch_cpu_init_f(). Allow an EVENT_DEBUG option which makes it
easier to see what is going on at runtime, but uses more code space.

Dynamic events allow the creation of a spy at runtime. This is not always
necessary, but can be enabled with EVENT_DYNAMIC if needed.

A 'test' event is the only option for now.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/Makefile

@@ -89,6 +89,8 @@ obj-y += malloc_simple.o
 endif
 endif
 
+obj-$(CONFIG_$(SPL_TPL_)EVENT) += event.o
+
 obj-$(CONFIG_$(SPL_TPL_)HASH) += hash.o
 obj-$(CONFIG_IO_TRACE) += iotrace.o
 obj-y += memsize.o