bios/boot: add bootargs support on netboot/sdcardboot to optionally specify r1/r2...

bios/boot: add bootargs support on netboot/sdcardboot to optionally specify r1/r2/r3/addr.

For example:
{
	"Image":  "0x40000000",
	"bootargs": {
		"r1":  "0x12345678",
	}
}

will copy Image to 0x40000000 and set r1 to 0x12345678.

By default, r1,r2,r3 are set to 0 and addr is the address if the last loaded image, so:

{
	"Image":         "0x40000000",
	"rootfs.cpio":   "0x40800000",
	"rv32.dtb":      "0x41000000",
	"emulator.bin":  "0x41100000",
}

is equivalent to:

{
	"Image":         "0x40000000",
	"rootfs.cpio":   "0x40800000",
	"rv32.dtb":      "0x41000000",
	"emulator.bin":  "0x41100000",
	"bootargs": {
		"r1":   "0x00000000",
		"r2":   "0x00000000",
		"r3":   "0x00000000",
		"addr": "0x00000000",
	}
}