Add Renesas R-Car Gen4 SCIF/HSCIF loader compatible SREC generation.
This is a regular U-Boot SPL SREC augmented with a short header which
describes where to store the received data and how much data to store.
This header is interpreted by the R-Car Gen4 BootROM SCIF/HSCIF loader.
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>