Bugfix for linker script

The previous commits needed more stack size than was previously allocated,
and changed the STACK_SIZE variable in configure.py accordingly.
Strangely, however, the linker still only allocated 2KB to the stack.
On further investigation, the line

    STACK_SIZE = DEFINED(__stack_size__) ? __stack_size__ : 0x800;

doesn't work, it always sets STACK_SIZE to 0x800. But changing it to

    STACK_SIZE = __stack_size__

works perfectly

Author: aquaticsarah

firmware/scripts/samd21g18a.ld

@@ -73,7 +73,7 @@ SRAM_SIZE = 0x8000;        /* 32kB, 32,768 bytes */
    * https://gcc.gnu.org/onlinedocs/gnat_ugn/Static-Stack-Usage-Analysis.html
    * https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
 */
-STACK_SIZE = DEFINED(__stack_size__) ? __stack_size__ : 0x800;
+STACK_SIZE = __stack_size__;
 
 /*
    Memory space definition.