Instead of user defined symbols in assembly files or C files,
use linker scripts to add heap and stack - this is inconsistent
with ARM std linker scripts
Linker files for LPC11U68 and LPC1549 updated as Mbed OS memory model
1. LPC1549
Has multiple memory banks, 4K bank used for stack exclusively and 16K for Heap
Its is mbed 2 only target hence 4K for stack.
2. LPC11U68
Added Heap and Stack sections in linker files. Resolved build issues.
StackTop calculation was not adjusted when ram size was increased by using al 3 ram regions.
This caused memory allocation failures althogh enough free heap was reported
--legacyalign, --no_legacyalign are deprecated from ARMC6 compiler, in order to
remove deprecated flags all linker files (GCC and IAR as well to have uniformity)
should strictly align to 8-byte boundary