Xilinx System debugger (XSDB) on an FSBL application does not allow c-code debug or for breakpoints to be placed in FSBL code.
This is due to flags which get set to optimize the code for size.
To resolve this issue, open the C/C++ Settings for the FSBL application.
Under "ARM v8 gcc compiler" -> Miscellaneous -> Other flags, note the following setting:
-c -fmessage-length=0 -MT"$@" -Os -flto -ffat-lto-objects
-c -fmessage-length=0 -MT"$@"