rkflashtool: support mask rom mode

add following commands for mask rom mode
 "rkflashtool l" for uploading (encrypted) DDR init code.
 "rkflashtool L" for uploading (encrypted) loader.

e.g.
 openssl rc4 -K 7c4e0304550509072d2c7b38170d1711 < 32_LPDDR2_300MHz_LPDDR3_300MHz_DDR3_300MHz_20140630.bin | rkflashtool l
 openssl rc4 -K 7c4e0304550509072d2c7b38170d1711 < u-boot.bin | rkflashtool L

for now, I confirmed that rk30usbplug.bin (on RK3188), rk32xxusbplug.bin
(on RK3288), and u-boot.bin for RK3288 can be run as loader.

XXX: rc4 encryption should be done in rkflashtool