Currently, building CentOS 6.2 or RHEL 6.2 EC2 images means that EC2 devices configured at /dev/sda will show up as /dev/xvde. There was a length discussion in RedHat's Bugzilla about it a: https://bugzilla.redhat.com/show_bug.cgi?id=729586, the end result being that they're adding a boot-time kernel parameter that you can set to get your emulated SCSI devices to map /dev/sda back to /dev/xvda.
How does one go about setting a boot-time kernel parameter when building an appliance, if it's even possible? I looked through the documentation and couldn't find any references.
I think one way to do this would be to simply modify your grub.conf config file in the post section and append it.
For instance, you could use sed to match the kernel line and add your new parameters at the end.
We don't have an explicit mechanism at present.
Does this work for you?
Edit: A really simplified example of how you could do it, you'd probably want to tweak it fairly significantly
sed -i -E 's/^\tkernel(.*)$/kernel\1 my.flag=1/' /etc/grub.conf
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