core: harmonizes the data type for the process ID

Instead of using differing integer types use kernel_pid_t for process
identifier. This type is introduced in a new header file to avoid
circular dependencies.

Author: OlegHahm

boards/avsextrem/drivers/avsextrem-smb380.c

@@ -44,7 +44,7 @@
 #include "lpc2387.h"
 
 
-uint8_t simple_pid;
+kernel_pid_t simple_pid;
 int16_t simple_buffer[4];
 
 volatile int16_t *ringBuff_X = NULL;

boards/msba2-common/drivers/include/uart0.h

@@ -1,6 +1,6 @@
 #ifndef __UART0_H
 #define __UART0_H
 
-extern int uart0_handler_pid;
+extern kernel_pid_t uart0_handler_pid;
 
 #endif /* __UART0_H */