ESXi is supported on our platforms, you still need to use our compatibility matrix to be sure about the compatibility of all the components so you can use this tool to check it out:
http://www.cisco.com/web/techdoc/ucs/interoperability/matrix/matrix.html
By being asked to press Ctrl+M you confirm that the embedded RAID controller is the one enabled, which confirms what you stated above (that the embedded RAID controller is not supported for ESXi installations):
http://www.cisco.com/c/en/us/td/docs/unified_computing/ucs/c/hw/C200M1/install/c200M1/RAID.html#pgfId-1014343 << You can confirm here the different key combinations for the different type of controllers.
Follow this guide to disable the embedded RAID controller:
http://www.cisco.com/c/en/us/td/docs/unified_computing/ucs/c/hw/C220/install/C220/raid.html#pgfId-1029510
After the embedded controller is disabled, go to the Server tab > BIOS> Advanced tab> Scroll down to the bottom and disable all the option ROMs for the PCIe and mezzanine slots EXCEPT for the slot where your hardware RAID controller will be installed and attempt the installation again with a Cisco customized version of the ESXi installer which will include the drivers.
HTH,
-Kenny