I want to boot our i.MX7 SABRE board from the onboard QSPI flash. I managed to activate the flash by reworking resistors as described in the QSPI NOR Flash section of the schematics. When booting via Serial Downloader, I can load and write bootloader binary image into QSPI flash.
However, the board does not boot from the QSPI device. I have removed the supplied SD card containing original u-boot and set BOOT_MODE switch SW3 to  (internal boot) and boot config switch SW2 to  as shown in schematics and printing on the board. It is only possible to load u-boot via serial download mode.
Even more, if SD card is attached, the board only boots from SD card, it is not possible to boot from serial downloader or QSPI flash. This is independent from Boot Config (SW2) and BOOT_MODE (SW3) settings with the exception that the board won't boot anymore if BOOT_MODE is set to  (Test Mode), but that's OK I think.
What can I do to manage booting from QSPI flash?