[rpi bsp] add cmdline options

[QIAO YANG]

Hi,

Some updates for you. I've found out that my previous problem with cmdline due to my minicom didn't wrap up the lines.... I've chosen the interface of vc to retrieve cmdline so that we don't need to parse the atag nor devicetree. Now we can use --video=...  to choose resolution and --console=... to choose console.

Some notes:

1. the rpi's cmdline can be setup by a plain-text, single-line file cmdline.txt.
2. I've implemented only 32-bpp. So the bpp is ignored. I may add other pixel drawing mode if necessary.
3. If no device connected to hdmi port, the display size would be 656x416. I use this as a probe.
4. --console=fbcons will select the fb console. Otherwise we will use the default option (serial console).
5. If we pass supported parameters to videocore, it will correct it and return the value configured. So I didn't port the part of code which select resolution from a list, but this lead to a behavior that we can pass some parameters like 12800x645  and get everything displayed.

I wonder if we have to limit the resolution in a mode_list.


here are precompiled binaries for tests.
https://www.dropbox.com/sh/9pvyf41zrjbx0hf/AAAIVJyCZPzXv0I1TnA5e7qUa?dl=0


Best wishes,
YANG Qiao
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