rockchip: mkimage: support packing optional second level boot-loader

Support packing optional second level boot-loader:

$ ./tools/mkimage -n rk3399 -T rksd -d \
  rk3399_ddr_800MHz_v1.24.bin:rk3399_miniloader_v1.19.bin out -v
Adding Image rk3399_ddr_800MHz_v1.24.bin
Size 116492(pad to 116736)
Adding Image rk3399_miniloader_v1.19.bin
Size 88060(pad to 88064)
Image Type:   Rockchip RK33 (SD/MMC) boot image
Init Data Size: 116736 bytes
Boot Data Size: 88064 bytes

Mainly parse init file and boot file from datafile option, copy them to
the image with 2KB alignment.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>