Can RasPi A+ boot from usb? All the articles I could find only speak about booting from an SD card.
No, you have to boot from the SD card.
The internal ROM first loads a small boot file (
bootcode.bin) from the SD card. This then examines the configuration (
cmdline.txt) and loads the
start.elf file (which can be specified in cmdline.txt). That then configures the base system (hardware peripherals etc).
start.elf then loads the kernel or another boot manager (such as uBoot).
The closest you can get is to use the SD card to boot the initial system and load the kernel, and have the root filesystem pointing to a USB device. Step-by-step details can be found here: https://samhobbs.co.uk/2013/10/speed-up-your-pi-by-booting-to-a-usb-flash-drive