reserved topmost 32 bytes of RAM used by IAP functions

NXP LPC176x/5x User Manual UM10360 Rev 4.1:
32.3.2.8 RAM used by IAP command handler
Flash programming commands use the top 32 bytes of on-chip RAM. The maximum stack 
usage in the user allocated stack space is 128 bytes and it grows downwards.

Author: 0x6d61726b

targets/TARGET_NXP/TARGET_LPC176X/device/TOOLCHAIN_GCC_CS/LPC1768.ld

@@ -20,13 +20,13 @@ SEARCH_DIR(.)
 
 /*
 ram ORIGIN: 8_byte_aligned(49 vect * 4 bytes) =  8_byte_aligned(0xC4) = 0xC8
-ram LENGTH: 32KB - 0xC8 = 0x7F38
+ram LENGTH: 32KB (RAM size) - 0xC8 (NIVT) - 32 (topmost 32 bytes used by IAP functions) = 0x7F18
 */
 MEMORY
 {
   rom (rx)  : ORIGIN = 0x00000000, LENGTH = 512K
 
-  ram (rwx) : ORIGIN = 0x100000C8, LENGTH = 0x7F38
+  ram (rwx) : ORIGIN = 0x100000C8, LENGTH = 0x7F18
 
   ram1(rwx) : ORIGIN = 0x2007C000, LENGTH = 16K
   ram2(rwx) : ORIGIN = 0x20080000, LENGTH = 16K