rpi-cmdline, rpi-u-boot-src: Support USB boot

The Raspberry Pi 4 board can boot the image from USB memory.
But current meta-raspberrypi supports only SD boot.

This commit allows switching SD boot to other device.
For example, if you want to use USB boot, please define
the following parameters in your conf/local.conf file.
CMDLINE_ROOT_PARTITION = "/dev/sda2"
BOOT_MEDIA = "usb"

v4: Update extra-build-config.md to explain more detail for new variable.

v3: Changed variable name to "BOOT_MEDIA" (was "BOOT_DEVTIPE").

v2: Set existing microSD parameter as default. User can overwrite
some parameters.

Signed-off-by: Harunobu Kurokawa <harunobu.kurokawa.dn@renesas.com>

recipes-bsp/bootfiles/rpi-cmdline.bb

@@ -9,7 +9,9 @@ inherit deploy nopackages
 CMDLINE_DWC_OTG ?= "dwc_otg.lpm_enable=0"
 
 CMDLINE_ROOT_FSTYPE ?= "rootfstype=ext4"
-CMDLINE_ROOTFS ?= "root=/dev/mmcblk0p2 ${CMDLINE_ROOT_FSTYPE} rootwait"
+CMDLINE_ROOT_PARTITION ?= "/dev/mmcblk0p2"
+
+CMDLINE_ROOTFS ?= "root=${CMDLINE_ROOT_PARTITION} ${CMDLINE_ROOT_FSTYPE} rootwait"
 
 CMDLINE_SERIAL ?= "${@oe.utils.conditional("ENABLE_UART", "1", "console=serial0,115200", "", d)}"
 

recipes-bsp/rpi-u-boot-scr/files/boot.cmd.in

@@ -1,4 +1,4 @@
 fdt addr ${fdt_addr} && fdt get value bootargs /chosen bootargs
-fatload mmc 0:1 ${kernel_addr_r} @@KERNEL_IMAGETYPE@@
-if test ! -e mmc 0:1 uboot.env; then saveenv; fi;
+fatload @@BOOT_MEDIA@@ 0:1 ${kernel_addr_r} @@KERNEL_IMAGETYPE@@
+if test ! -e @@BOOT_MEDIA@@ 0:1 uboot.env; then saveenv; fi;
 @@KERNEL_BOOTCMD@@ ${kernel_addr_r} - ${fdt_addr}

recipes-bsp/rpi-u-boot-scr/rpi-u-boot-scr.bb

@@ -9,9 +9,12 @@ INHIBIT_DEFAULT_DEPS = "1"
 
 SRC_URI = "file://boot.cmd.in"
 
+BOOT_MEDIA ?= "mmc"
+
 do_compile() {
     sed -e 's/@@KERNEL_IMAGETYPE@@/${KERNEL_IMAGETYPE}/' \
         -e 's/@@KERNEL_BOOTCMD@@/${KERNEL_BOOTCMD}/' \
+        -e 's/@@BOOT_MEDIA@@/${BOOT_MEDIA}/' \
         "${WORKDIR}/boot.cmd.in" > "${WORKDIR}/boot.cmd"
     mkimage -A ${UBOOT_ARCH} -T script -C none -n "Boot script" -d "${WORKDIR}/boot.cmd" boot.scr
 }