You can use write_cfgmem
from Vivado tcl command or srec_cat
from srecord package.
Using write_cfgmem
vivado -mode tcl
Vivado% write_cfgmem -format MCS -size 16 -interface SPIx4 -loadbit "up 0 /path/to/a.bit" /path/to/output.mcs
Or save it in a .tcl
and run it.
Using srec_cat
$ srec_cat -Output_Block_Size=16 /tmp/a.bit -Binary -o /tmp/a.hex -Intel