Add a simple initcall declaration framework

This commit adds a framework that, hopefully, would allow for strong
module decoupling of the code of milliboot. It introduces a set of
macros and linker scripts directive that allow user to define so called
initcalls, which are just simple functions doing initialization tasks.

To the best of my knowledge, the idea originated in Linux kernel and
was borrowed by the barebox project, form which this implementations
borrowed all the crucial pieces.

For examples of the usage see milliboot/leds.c

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>