cheatsheet.md: Re-write QEMU invocation example

[amboar]

The commandline provided had several issues:

• The -dtb option was redundant as we boot with the cuImage, and caused some kernel configuration to be skipped which in turn ended in a kernel panic
• -append argument was redundant with respect to the devicetree
• The file paths used typically required the user to change up out of the build directory after bitbaking obmc-phosphor-image

The patch addresses all of the above.

Signed-off-by: Andrew Jeffery andrew@aj.id.au

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