native: -DNATIVE_INCLUDES if using NATIVEINCLUDES

It can be useful to know if the current compilation builds the native
board module, because e.g. then we must not include `sys/types.h`.

This diff adds `-DNATIVE_INCLUDES` to the make variable
`NATIVEINCLUDES`.
 core: Provide ssize_t in a common place

 Now you can include `kernel_types.h` if you need the `ssize_t`, without
 facing problems due to the terrible MSP toolchain.
stm32f0: be UART0 aware
stm32f4: be UART0 aware
stm32f3: be UART0 aware
stm32f1: unify UART0 use
stm32f*: rename STDIO buffer and enable getchar with UART0
Merge pull request #1471 from thomaseichinger/impl_uart0

cpu: make stm32f{0,1,3,4} comply to UART0 module
boards:mega2560: add debug and debug-server targets
Merge pull request #1696 from N8Fear/mega2560-debug

boards:mega2560: add debug and debug-server targets
drivers: import PIR motion sensor driver

Includes an application for manual testing.
Merge pull request #1714 from LudwigOrtmann/add_driver_pir

drivers: import PIR motion sensor driver
make: add objdump target
make: .PHONY missing targets from Makefile.include
Merge pull request #1723 from LudwigOrtmann/issue-1719

make: add objdump target + .PHONY missing targets from Makefile.include
Merge pull request #1698 from Kijewski/ssize_t-for-all

core: Provide ssize_t in a common place
pkg/Makefile.{http,git}: Minor issues resolved.

The were two recipes for clean. One of them is now "dist-clean"
make: 1571 fixup: dont create existing directories

This fixes an error which was introduced by commit
346313bf07304c5636739d028c7b70acb4b507f9

The timestamp of directories is updated when a file inside a directory
is changed.
Therefore, make decides a target needs to be rebuilt, whenever that
target depends on its parent directory, because the directory is
always newer than the file inside.

http://www.gnu.org/savannah-checkouts/gnu/make/manual/html_node/Prerequisite-Types.html

    Occasionally, however, you have a situation where you want to
    impose a specific ordering on the rules to be invoked without
    forcing the target to be updated if one of those rules is
    executed. In that case, you want to define order-only
    prerequisites. Order-only prerequisites can be specified by
    placing a pipe symbol (|) in the prerequisites list: any
    prerequisites to the left of the pipe symbol are normal; any
    prerequisites to the right are order-only:

         targets : normal-prerequisites | order-only-prerequisites
make: optimize archive creation/modification

Only add/replace changed members to the archive.
Merge pull request #1462 from backenklee/smallPR

pkg/Makefile.http: There were two recipes for clean.