fix write for microcontroler with RAM size less or equal to 32K

[trabucayre]

This fix is due to a problem to flash STM32F410 with large .bin (for example nuttx.bin). The result is :
st-flash 1.4.0-8-g1700e6a
2017-09-16T17:15:20 INFO src/common.c: Loading device parameters....
2017-09-16T17:15:20 INFO src/common.c: Device connected is: F410 device, id 0x10006458
2017-09-16T17:15:20 INFO src/common.c: SRAM size: 0x8000 bytes (32 KiB), Flash: 0x20000 bytes (128 KiB) in pages of 16384 bytes
2017-09-16T17:15:20 INFO src/common.c: Attempting to write 57728 (0xe180) bytes to stm32 address: 134217728 (0x8000000)
Flash page at addr: 0x0800c000 erasedEraseFlash - Sector:0x3 Size:0x4000
2017-09-16T17:15:21 INFO src/common.c: Finished erasing 4 pages of 16384 (0x4000) bytes
2017-09-16T17:15:21 INFO src/common.c: Starting Flash write for F2/F4/L4
2017-09-16T17:15:21 INFO src/flash_loader.c: Successfully loaded flash loader in sram
enabling 32-bit flash writes
size: 32768
2017-09-16T17:15:24 ERROR src/flash_loader.c: flash loader run error
2017-09-16T17:15:24 ERROR src/common.c: stlink_flash_loader_run(0x8000000) failed! == -1
stlink_fwrite_flash() == -1

This issue is related to the buffer size : in src/common.c this size is fixed to 32KB.
To be able to use this size the target needs to have a RAM greater this value.
Indeed the flash loader is located at the begin of the RAM, on F4, it is written in SRAM at 0x2000000 and use 36b, then fl->buf_addr is fixed to 0x20000024.

For a F410RB with 32KB of RAM it's consequently not possible to load a buffer with the default size.

This patch check if the target has at least 32KB of RAM. If not the size is divided by 2.

[xor-gate]

LGTM