base-hw: rework 'run/env', enable '--target uboot'

ref #601
ref #598
base-hw: provide SIGNAL service in nested_init.run

ref #601
base: make signal framework usable for base-hw

ref #641
base-hw: destroy signal contexts, generic signal.h

fix #641
timer & hw & omap4: compile bug in Platform_timer

fix #652
base-hw: useless default assignment in syscalls.h

fix #588
base-hw & arm: bugs when zero-filling BSS

fix #654
base-hw: rebase onto base lib merge of @nfeske

fix #18