lib: Update samd21 code with local changes

A minor change is necessary to work with GCC's -flto and to work with
common bootloaders.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

lib/samd21/samd21a/gcc/gcc/samd21g18a_flash.ld

@@ -35,7 +35,7 @@ SEARCH_DIR(.)
 /* Memory Spaces Definitions */
 MEMORY
 {
-  rom      (rx)  : ORIGIN = 0x00000000, LENGTH = 0x00040000
+  rom      (rx)  : ORIGIN = 0x00000000 + FLASH_START , LENGTH = 0x00040000 - FLASH_START
   ram      (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000
 }