The steps below were tested to work on Devuan 1.0 and were used to update an HP ProLiant DL380e Gen8.

Based on the blogpost Eren Türkay - Creating Bootable USB For HP Blades (Service Pack).

Tools needed:

• fdisk from util-linux (tested with 2.25.2)
• mkfs.fat (tested with 3.0.27 (2014-11-12))
• syslinux (tested with 6.03)
• ms-sys (tested with 2.5.3)
• gettext (for msgfmt)

Change /dev/sdd in the below example commands to the device name of the USB device.

Quick check: insert the USB device, execute the 'dmesg' command, and check the last lines for the /dev/sdX name (where X is the letter of the device.

Get the HP SPP ISO from the hpe.com website. Tested with SPP 2017.04.0 (871795_001_spp-2017.04.0-SPP2017040.2017_0420.14.iso)

As ms-sys does not seem to be available in Devuan 1.0 packages we need to get and compile the program.
Get ms-sys-2.5.3.tar.gz from the ms-sys website.

Extract:

tar xzvf ms-sys-2.5.3.tar.gz

Compile:

cd ms-sys-2.5.3
make

The ms-sys executable can now be found in the ./bin/ directory.
You can install it with 'make install' but I did not and used it from the compile directory instead.

# zero-out mbr
ms-sys -z /dev/sdx

# create msdos label
parted -s /dev/sdx mklabel msdos

# write syslinux MBR provided by ms-sys 
ms-sys -s /dev/sdx

Start fdisk on the USB drive:

fdisk /dev/sdd

Enter the following commands in fdisk (partial instruction):

1. n one primary partition, using all space
2. t and set it to type 'c' (w95 fat)
3. a to set it bootable
4. w to write changes

Format the partition as FAT32 (-F 32) and volume name USB (-n USB):

/sbin/mkfs.vfat -F 32 -n USB /dev/sdd1

Install syslinux on the USB drive:

/usr/bin/syslinux -i /dev/sdd1

Create the directory iso and usb under /mnt/ to mount the ISO and the USB drive:

mkdir /mnt/iso
mkdir /mnt/usb

Mount the iso and the USB drive.

mount 871795_001_spp-2017.04.0-SPP2017040.2017_0420.14.iso /mnt/iso
mount /dev/sdd1 /mnt/usb

Copy the files to the USB drive:

# Set source and destination variables.
export HP_SOURCE="/mnt/iso"
export HP_USB_DISK="/mnt/usb"

# Copy all the files from the ISO to the USB drive.
rsync -rltDuvh --progress $HP_SOURCE/ $HP_USB_DISK/

# Copy files required for booting from the /system/ directory on the ISO to the base of the USB drive.
cp -rf $HP_SOURCE/system/initrd.img $HP_USB_DISK
cp -rf $HP_SOURCE/system/vmlinuz $HP_USB_DISK
cp -rf $HP_SOURCE/system/*.c32 $HP_USB_DISK
cp -rf $HP_SOURCE/system/*.jpg $HP_USB_DISK
cp -rf $HP_SOURCE/system/blacklist $HP_USB_DISK
cp -rf $HP_SOURCE/system/sample.msg $HP_USB_DISK
cp -rf $HP_SOURCE/usb/syslinux.cfg $HP_USB_DISK

# HP's vesamenu file gives error. Copy vesamenu from system.
cp -f /usr/lib/syslinux/modules/bios/vesamenu.c32 $HP_USB_DISK

# Syslinux 6.03 needs extra files in addition to it's vesamenu.c32
cp -f /usr/lib/syslinux/modules/bios/libcom32.c32 $HP_USB_DISK/
cp -f /usr/lib/syslinux/modules/bios/libutil.c32 $HP_USB_DISK/

# Sync the changes to disk.
sync

Unmount the ISO and the USB drive:

umount /mnt/iso
umount /mnt/usb

Now the USB drive should be bootable and ready to use for firmware upgrades.