Hi,
I had also faced the same problem with IBM x3650 M4 (7915) Model.
1. Downloaded latest firmware from site.
2. Updated the system.
3. Set the boot order to legacy Only
4. Burnt IBM 5.5.0U1 ISO to CD.
5. Press F12 to select boot Device
6. Select "Legacy Only ", the first option on top with [ ]. Use space bar to select and then press enter to boot.
7. System booted normally showing the virtual drives and NIC's. Gone ahead with CD boot and installed ESXI 5.5U1 successfully. Please check step 6 carefully. otherwise it does not boot.
Thanks
Samod Babu