soc/software/bios/boot: add vexriscv workaround

Flushing icache was working correctly on previous version of Vexriscv, understand
why it's no longer the case.

litex/soc/software/bios/boot.c

@@ -25,7 +25,10 @@ static void __attribute__((noreturn)) boot(unsigned int r1, unsigned int r2, uns
 	uart_sync();
 	irq_setmask(0);
 	irq_setie(0);
+/* FIXME: understand why flushing icache on Vexriscv make boot fail  */
+#ifndef __vexriscv__
 	flush_cpu_icache();
+#endif
 	flush_cpu_dcache();
 #ifdef L2_SIZE
 	flush_l2_cache();