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.

core/include/kernel_types.h

@@ -11,6 +11,24 @@
 
 #include <stdint.h>
 #include <inttypes.h>
+#include <limits.h>
+
+#ifndef NATIVE_INCLUDES
+#   include <stddef.h>
+#   include <sys/types.h>
+
+#   ifndef SSIZE_MAX
+#       ifdef _POSIX_SSIZE_MAX
+#           define SSIZE_MAX _POSIX_SSIZE_MAX
+#       else
+#           define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
+#       endif
+#   endif
+
+#   if defined (MODULE_MSP430_COMMON) || defined (MODULE_ATMEGA_COMMON)
+        typedef signed ssize_t;
+#   endif
+#endif
 
 /**
  * @def MAXTHREADS

cpu/atmega_common/include/sys/types.h

@@ -10,4 +10,3 @@
 #include <stddef.h>
 
 typedef int16_t suseconds_t;
-typedef size_t ssize_t;

cpu/msp430-common/include/msp430_types.h

@@ -12,6 +12,9 @@
 #include <sys/_types.h>
 #include <errno.h>
 
+/** defining signed type for size_t */
+#include "kernel_types.h"
+
 #ifndef EINVAL
 /**
  * @brief defines EINVAL if MSP430 toolchain is too old to provide it itself
@@ -28,9 +31,6 @@
 #define EOVERFLOW   (65)
 #endif
 
-/** defining signed type for size_t */
-typedef _ssize_t ssize_t;
-
 typedef unsigned long time_t;
 
 struct timespec {