The bootloader used to boot Ironic nodes in UEFI boot mode during
inspection when iPXE is enabled has been changed from ipxe.efi to
snponly.efi. This is in line with the default UEFI iPXE bootloader used
in Ironic since the Xena release. The bootloader may be changed via
ironic_dnsmasq_uefi_ipxe_boot_file.

Note that snponly.efi was not available via in the ironic-pxe image
prior to I79e78dca550262fc86b092a036f9ea96b214ab48.

Related-Bug: #1959203

Change-Id: I879db340769cc1b076e77313dff15876e27fcac4