Fix system crash when using bossac on UART

The crash is caused by an attempt to write to flash address space
(0x804000). The address has been changed to the middle of SRAM
(0x20004000). This bug does not show up when using USB CDC.

Author: GiedriusM

src/FlashFactory.cpp

@@ -52,12 +52,12 @@ FlashFactory::create(Samba& samba, uint32_t chipId)
     //
     case 0x10010000:
 	case 0x10010100:
-        flash = new NvmFlash(samba, "ATSAMD21J18A", 0x000000, 4096, 64, 1, 16, 0x804000, 0x20008000, 0x41004000, true);
+        flash = new NvmFlash(samba, "ATSAMD21J18A", 0x000000, 4096, 64, 1, 16, 0x20004000, 0x20008000, 0x41004000, true);
         // 0x41004000 == Base address for the NVMCTRL module
         break;
 
     case 0x10010005:
-        flash = new NvmFlash(samba, "ATSAMD21G18A", 0x000000, 4096, 64, 1, 16, 0x804000, 0x20008000, 0x41004000, true);
+        flash = new NvmFlash(samba, "ATSAMD21G18A", 0x000000, 4096, 64, 1, 16, 0x20004000, 0x20008000, 0x41004000, true);
         break;
 
     //