video/fb: kernel build framebuffer support
This supports running fb demo app in kernel build with new
config `rv-virt/kfb64`. The demo shows colored rectangles
in graphic window with the following console logs:
```
nsh> cat /proc/version
NuttX version 12.4.0 1ea10ddacc-dirty May 3 2024 07:03:59 rv-virt/kfb64
nsh> fb
VideoInfo:
fmt: 13
xres: 640
yres: 480
nplanes: 1
PlaneInfo (plane 0):
fbmem: 0x80218010
fblen: 1228800
stride: 2560
display: 0
bpp: 32
Mapped FB: 0xc2000010
0: ( 0, 0) (640,480)
1: ( 58, 43) (524,394)
2: (116, 86) (408,308)
3: (174,129) (292,222)
4: (232,172) (176,136)
5: (290,215) ( 60, 50)
Test finished
```
Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>
041ef1d9ea