core: remove unnecessary tests in sched.c

The scheduling gets activated by `kernel_init()` calling
`cpu_switch_context_exit()`. Before this `sched_run()` won't be called.
When it gets called, at least the main thread and the idle thread are
spawned. The idle thread won't die / get killed. So there always is at
least one thread in `runqueue_bitcache`.

Closes #19.
cpu - cortex-m3: moved crash.c to cortex-m3_common
Merge pull request #1327 from haukepetersen/fix_cortexm3_crash

cpu - cortex-m3: moved crash.c to cortex-m3_common
Merge pull request #1306 from Kijewski/issue-19

core: remove unnecessary tests in sched.c
tests: fixed Makefile for test_ipc_pingpong
Merge pull request #1328 from haukepetersen/fix_test_ipcpingpong

tests: fixed Makefile for test_ipc_pingpong
make: %s/PROJECT/APPLICATION/g Part III
Merge pull request #1330 from LudwigOrtmann/projects_removal

make: %s/PROJECT/APPLICATION/g Part III
make: detect their module name automatically

For many modules the `Makefile` contains a line like
```
MODULE:=$(shell basename $(CURDIR))
```
This conclusively shows that we do not have to set the module name
manually.

This PR removes the need to set the module name manually, if it is the
same as the basename. E.g. for `…/sys/vtimer/Makefile` the variable
make `MODULE` will still be `vtimer`, because it is the basename of the
Makefile.
make: easifier usage of module subdirectories

Many modules have subdirectories. Often these subdirectories should only
be included under certain circumstances. Modules that use submodules
currently need to use this pattern:

```make
DIRS = …

all: $(BINDIR)$(MODULE).a
   @for i in $(DIRS) ; do $(MAKE) -C $$i ; done ;

include $(RIOTBASE)/Makefile.base

clean::
   @for i in $(DIRS) ; do $(MAKE) -C $$i clean ; done ;
```

This PR moves the `all:` and `clean::` boilerplate into `Makefile.base`.
make: refactor make system (parallelism, deduplication)

Almost everything was build sequentially in RIOT, because we employed
explicit for-loops to build directories (DIRS). This PR makes our make
system use normal dependencies to build directories.

All our compiling rules were duplicated, once for the application, once
for modules. This PR makes the application a normal module, removing
this duplication.
make: fix PROJDEPS
make: make DIRS usable for applications
make: create dependencies as side effect