[U-BOOT][MKIMAGE][PX30] Use specific mkimage for px30
After passing to u-boot-seco-rk branch next, using the usual mkimage will compile seco_boot.scr in a way that is not readable for u-boot, resulting in a non bootable device
After passing to u-boot-seco-rk branch next, using the usual mkimage will compile seco_boot.scr in a way that is not readable for u-boot, resulting in a non bootable device