read configurations from include/config.h and revise symmap targets
sampling: syntax tweak and make static to internal functions
sampling: conditional build for KDB
sort: rewrite by using routines taken from FreeBSD
sampling: make internal function static
mk: trivial indent for commands
sampling: output format tweaks
mk: mention the limitation of configurations
unify the license notice
debug_uart: hide internal functions
README: fix typo
sampling: feature depends on CONFIG_KPROBES
build: include <config.h> from CPPFLAGS rather than individual
config: resolve depenedency of the existing configurations

As far as we know, CONFIG_SYMMAP depends on CONFIG_KPROBES, and we
attempt to perform post-filtering processing in the end of config.h
kernel: conditional build of KDB
Add SCB_ICSR_RETTOBASE which can be used to determinate
Whether there are preempted active exception.
Add schedule_select whcih can be used to select running
thread.
Support nested IRQ and increase IRQ performance by
doing context switch at necessary condition.
To save space, implement default device IRQ handler by weak alias.
And use IRQ_HANDLER to implement used IRQ handler.
Move irq_init and irq_disable to beginning of main.
It is needed to avoid unexcepted device IRQ.