cc2538: Move the stack out of .bss

The initialization code clearing .bss is allowed to use the stack, so
the stack can not be in .bss, or this code will badly fail if it uses
the stack.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>

cpu/cc2538/cc2538.lds

@@ -106,6 +106,11 @@ SECTIONS
         _ebss = .;
     } > SRAM
 
+    .stack (NOLOAD) :
+    {
+        *(.stack)
+    } > SRAM
+
 #if (LPM_CONF_MAX_PM==2) && (LPM_CONF_ENABLE != 0)
     .nrdata (NOLOAD) :
     {

cpu/cc2538/startup-gcc.c

@@ -89,7 +89,7 @@ void uart1_isr(void) WEAK_ALIAS(default_handler);
 #endif
 /*---------------------------------------------------------------------------*/
 /* Allocate stack space */
-static unsigned long stack[512];
+static unsigned long stack[512] __attribute__ ((section(".stack")));
 /*---------------------------------------------------------------------------*/
 /* Linker construct indicating .text section location */
 extern uint8_t _text[0];