Keep the compiled '.bin' file to remove the need to compile it when
flashing. This remove the need to have the toolchain when flashing so
allow compiling and flashing with `BUILD_IN_DOCKER=1` without a local
toolchain.
Even if it ends up storing a binary, the file is only 34 bytes.
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.
