The Appollo3 targets require dummy sections in stack and heap regions.
The stack dummy section does not contain any symbols. It is only used
for the linker to calculate the size of the stack sections and assign
values to stack symbols later.
The heap dummy region is used to identify the beginning of available dynamic memory.

The heapsize was incorrectly calculated.
This fixes it by subtracting the Stack size, any memory chunks allocated
before the start of the application (for vectors and/or crash report), and
finally the size of the application from the total RAM size.

The stack start address should be the top of the RAM which is also fixed.

Workaround a bug where the boot stack size configuration option is not
passed on to armlink, the Arm Compiler's linker. Prefer
MBED_CONF_TARGET_BOOT_STACK_SIZE if present, as this is what the
configuration system should provide. Fall back to MBED_BOOT_STACK_SIZE
if MBED_CONF_TARGET_BOOT_STACK_SIZE is not defined, as in the case of
buggy tools. If both MBED_CONF_TARGET_BOOT_STACK_SIZE and
MBED_BOOT_STACK_SIZE are not defined, then we fall back to a hard-coded
value provided by the linkerscript. See
#13474 for more information.