Get OBJDUMP from the environment instead of hardwriting the value.
This is a prerequisite to allow using `objdump` when building from docker
when not having the `arm` toolchain installed.
- fcfield is located in memory at 0x400-0x40f. Its content is only read
upon reset, therefore if in presence of a bootloader and multiple
applications, the fcfield will only be read when the bootloader
is loaded. As long as we flash at IMAGE_OFFSET > 0x410 we do not
care about the fcfield content since it won't get overwritten.
