changed parameters form `kernel_pid_t` to `int`
Fix thread_yield by avoiding the (too) early re-enablement of IRQ,
that is: before the newly selected thread's context is totally restored
Prevent the compiler to add an unwanted prologue in thread_yield()
Fix the race condition when an interruption occured at the beginning
of thread_yield(), i.e.: just after SR(R2) was pushed but before
the rest of the suspended thread's context was pushed!
Fixed comments
tests: don't build test/libfixmath on Travis CI

Very often the build unexplainably hangs.
Merge pull request #1648 from Kijewski/no-test-libfixmath

tests: don't build test/libfixmath on Travis CI
Merge pull request #1561 from BytesGalore/fix_fd_parameter_warnings

sys/posix: changed parameters for `fd` from `kernel_pid_t` to `int`
transport_layer: Splitting UDP and TCP

Currently, the tcp and udp implementations are bound to each other in a
module called *destiny*. Thus, when using only one of them then the
other one gets also compiled into the binary and initialized,
which results in unnecessary RAM usage and workload for the CPU.

The approach in this PR defines a common module named *socket_base*,
which contains functions used by the posix layer. Compiled by it's own,
those functions return negative error codes, to symbolize upper layers
that they are not supported. When also including the modules *udp* or
*tcp* respectively, functions from *socket_base* get overwritten with the
correct functionality.

Defining *udp* or *tcp* in a Makefile also includes *socket_base*.
Defining *pnet* in a Makefile also includes *socket_base*.
Merge pull request #1508 from cgundogan/transport_layer_refactoring

transport_layer: Splitting UDP and TCP
drivers-lm75a: use sizeof array instead of reg_size
lint: reduce scope

Fix some lint warnings of kind
```
style (variableScope): The scope of the variable '…' can be reduced
```
cpu: lpc_common: reduce indentation level in iap.c
Merge pull request #1565 from Kijewski/scope

lint: reduce scope
Merge pull request #1618 from rousselk/msp430-fix-thread-yield

Fix thread_yield() on MSP430 platforms
dns: added dns client