Add uenvcmd support

Change root from static to dynamic with default (nandb) support.
Change kernel from static to dynamic with default (/uImage) support.
Create uenvcmd support for initrd on nand.

uEnv.txt
root=/dev/nandc
kernel=/uImage.3.4.75-sun71
uenvcmd=ext4load nand 1:0 0x44000000 /boot/uInitrd; bootm 0x48000000 0x44000000
extraargs=hdmi.audio=EDID:0 disp.screen0_output_mode=EDID:1280x800p60 panic=10 consoleblank=0 sunxi_no_mali_mem_reserve sunxi_g2d_mem_reserve=0 sunxi_ve_mem_reserve=0 sunxi_fb_mem_reserve=16
machid=10bb