ports/stm32f4: Keep bootloader within first 32K

In order to free up sectors 2 and 3 for the application, limit the
bootloader to the first 16K sector, config to the next 16K. We keep
the application still starting at 0x10000 so that 0x8000 and 0xC000
sectors can be used by the application for settings/config storage.

ports/stm32f4/linker/stm32f4_boot.ld

@@ -44,8 +44,8 @@ _Min_Stack_Size = 0x400; /* required amount of stack */
 MEMORY
 {
   RAM (xrw)     : ORIGIN = 0x20000000, LENGTH = 64K - 4 /* reserve 4 bytes for double tap */
-  FLASH (rx)    : ORIGIN = 0x08000000, LENGTH = 31K
-  CONFIG (rx)   : ORIGIN = 0x08008000 - 1024, LENGTH = 1024
+  FLASH (rx)    : ORIGIN = 0x08000000, LENGTH = 15K
+  CONFIG (rx)   : ORIGIN = 0x08004000 - 1024, LENGTH = 1024
 }
 
 /* Define output sections */