I had a look at what the spec says and what pxelinux and u-boot-linaro do in practice.

== pxelinux ==

Looking at pxelinux 4.04 as found in Ubuntu oneiric syslinux source package version 2:4.04+dfsg-1ubuntu1.

syslinux/core/fs/pxe/pxe.c:network_init() calls into the PXE ROM to get the DHCPACK packet with pxe_get_cached_info(2) (2 is BOOTP_REPLY according to the PXE spec and I confirmed this with a wire capture which shows 0x02 as the first byte of the BOOTP packet/UDP payload); the MAC type is then derived from the struct bootp_t hardware field and is always 1 for Ethernet (0x01 as the second byte of the BOOTP packet / UDP payload) and finally the actual MAC address is read from the macaddr field.

For completeness, syslinux/core/fs/pxe/pxe.h defines a bootp packet as follow:
struct bootp_t {
    uint8_t  opcode;        /* BOOTP/DHCP "opcode" */
    uint8_t  hardware;      /* ARP hreadware type */
[...]
    uint8_t  macaddr[16];   /* Client MAC address */

In syslinux/core/fs/pxe/pxe.c:make_bootif_string(), the  BOOTIF string is constructed as follows by pxelinux:
    dst += sprintf(dst, "BOOTIF=%02x", MAC_type);
    src = MAC;
    for (i = MAC_len; i; i--)
        dst += sprintf(dst, "-%02x", *src++);

and MAC_str just points at BOOTIFStr+7 (skipping "BOOTIF=")

Finally, pxe_load_config() in pxe.c attempts loading the following files:
 * the DHCP provided filename
 * pxelinux.cfg/ + UUID (if set in a DHCP option of the reply)
 * pxelinux.cfg/ + MAC_str (MAC type and MAC address bytes in hexa separated by dashes)
 * pxelinux.cfg/ + IP based strings (IP in hex without separators, getting shorter and shorter and less specific)
 * pxelinux.cfg/ + "default"

== u-boot-linaro ==

Looking at u-boot-linaro-stable 605c08e8b6f7b7e4b9d17b6cb367fb27ce1c511c.

u-boot-linaro/common/cmd_pxecfg.c:get_pxecfg() tries:
 * pxelinux.cfg/ + UUID (set in pxeuuid u-boot env)
 * pxelinux.cfg/ + MAC address; the MAC address is looked for in the format_mac_pxecfg() function which tries the ethaddr u-boot env and falls back to usbethaddr, then converts all colons to dashes
 * pxelinux.cfg/ + IP based strings (identical results to pxelinux -- sprintf(ip_addr, "%08X", ntohl(NetOurIP)))
 * pxelinux.cfg/ + "default"

However by default the bootp/dhcp commands will attempt to load the DHCP supplied filename, so that should result in a similar behavior to the pxelinux one.

The main differences in the u-boot implementation are:
 * UUID is set by the config / end-user in u-boot env instead and sent in DHCP requests rather than coming from DHCP replies
 * MAC address doesn't come from the packet but from the env
 * MAC type prefix is missing, should be read from the DHCP/BOOTP reply

== Spec ==

What the Intel PXE spec says is that:
 * UUID comes from SMBIOS tables
 * UUID should be sent in the DHCP requests
 * only the DHCP/BOOTP provided filenames should be tried, nothing more
 * doesn't say anything about where the MAC address should be obtained from or using a MAC type; this is pxelinux specific


But in reality, pxelinux is split in two stages: pxelinux.0 and then the config file it downloads; what u-boot emulates is what stage 1 does to download the stage 2 config and files that the config refers to.

So looking at what's missing, ideally u-boot would use the data from the dhcp/bootp reply rather than from the env, but this is painful to do in separate u-boot commands (dumping packets in the u-boot environment or storing them in a configured memory location would suck), hence the easiest way to match pxelinux is probably to just hardcode a 01- prefix before the MAC address requests.