configs: meson64: Ensure commands for boot targets are available

configs: meson64: Ensure commands for boot targets are available

When populating BOOT_TARGET_DEVICES on Amlogic Meson platforms, verify
that the shell commands for accessing the relevant media have been
enabled in the build configuration.

This was already being done for USB, NVME and SCSI devices, but not
for MMC devices or network boot. This meant that configuring U-Boot
without the networking or MMC subsystems/drivers would cause an error
to be thrown during preprocessing.

Signed-off-by: Callum Parsey <callum@neoninteger.au>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20241012080340.2596-1-callum@neoninteger.au
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>