Put some code into the first 16K of flash

This basically shrinks the remaining size of flash in the portion
that goes after the internal flash drive.

Comparison of the overall flash usage:

BEFORE:
```
LINK build-PYBV10/firmware.elf
   text	   data	    bss	    dec	    hex	filename
 228412	    392	  28104	 256908	  3eb8c	build-PYBV10/firmware.elf
```
AFTER:
```
LINK build-PYBV10/firmware.elf
   text	   data	    bss	    dec	    hex	filename
 228416	    392	  28104	 256912	  3eb90	build-PYBV10/firmware.elf
```

and the section sizes BEFORE:
```
2885 >objdump -h build-PYBV10/firmware.elf 

build-PYBV10/firmware.elf:     file format elf32-little

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .isr_vector   00000188  08000000  08000000  00008000  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  1 .text         00037ab4  08020000  08020000  00010000  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  2 .data         00000188  20000000  08057ab4  00048000  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  3 .bss          000025c8  20000188  08057c3c  00048188  2**2
                  ALLOC
  4 .heap         00004000  20002750  08057c3c  0004a750  2**0
                  ALLOC
  5 .stack        00000800  20006750  08057c3c  0004e750  2**0
                  ALLOC
  6 .ARM.attributes 00000035  00000000  00000000  00048188  2**0
                  CONTENTS, READONLY
  7 .comment      00000070  00000000  00000000  000481bd  2**0
                  CONTENTS, READONLY
```
and AFTER:
```
2889 >objdump -h build-PYBV10/firmware.elf 

build-PYBV10/firmware.elf:     file format elf32-little

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .isr_vector   000033e4  08000000  08000000  00008000  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .text         0003485c  08020000  08020000  00010000  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  2 .data         00000188  20000000  0805485c  00048000  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  3 .bss          000025c8  20000188  080549e4  00048188  2**2
                  ALLOC
  4 .heap         00004000  20002750  080549e4  0004a750  2**0
                  ALLOC
  5 .stack        00000800  20006750  080549e4  0004e750  2**0
                  ALLOC
  6 .ARM.attributes 00000035  00000000  00000000  00048188  2**0
                  CONTENTS, READONLY
  7 .comment      00000070  00000000  00000000  000481bd  2**0
                  CONTENTS, READONLY
```