To allow overriding of the boot stack size from the Mbed configuration
system, consistently use MBED_CONF_TARGET_BOOT_STACK_SIZE rather than
MBED_BOOT_STACK_SIZE.
Fixes#10319
Since mbed-os 5.4.3, something increased foot print of mbed-os and the applications that were barely fitting in started to spill.
IAR toolchain for LPC176x target family is set to use 2 RAM regions (32K each). RAM region
2 is being used for ETH/USB and 1 is being used for vector table, stack/heap/static data.
In this commit we have decreased heap size allocation from 8K to 7K so that the is more room for stack and static data.