Bootloader modifications for generic framebuffer console support on i386
and amd64 where VESA VBE 2.0+ is available.

* Add helper library and stubs to invoke VBE bioscalls.
* Bump HEAP_START as we w

Bootloader modifications for generic framebuffer console support on i386
and amd64 where VESA VBE 2.0+ is available.

* Add helper library and stubs to invoke VBE bioscalls.
* Bump HEAP_START as we were already dangerously close to our limits.
* bootdataseg now allows access > 16MB so in the future we can scribble
on the framebuffer.
* Pass BTINFO_FRAMEBUFFER parameters to kernel when configured.
* VBE modes are configured with the new 'vesa' command. Usage, when present:
* vesa {enabled|disabled}
Enable / disable linear framebuffer, default mode is 640x480x8. May
be changed in the future to determine mode based on VBE/DDC where
available.
* vesa list
List modes supported by the firmware that meet the following criteria:
* linear framebuffer
* packed pixel or direct colour mode
* vesa {modenum|modestr}
Enable specific VBE mode. The mode can be specified either as a
VBE mode number (eg. 0x101) or as a string (eg. 800x600x16).

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