Makefile: check if submodules are current, warn if not
Makefile: check if submodules are current, warn if not

Make litex-submodules also check if submodules are "current" (ie,
match the sha1 indicated in the parent repository -- timvideos/litex-buildenv)
and if not, emit a warning that they are out of date.

NOTE: we do _not_ force update in the Makefile, as this would make it
difficult to test changes inside submodules by building at top level
(eg, building new litex firmware, gateware, etc).

Possible future enhancements:
1.  Bail out of makefile (eg, "exit 1") if a variable indicating "development"
    is not set;

2.  Forcibly run "git submodule update --init --recursive" if found to be
    out of date, unless a variable indicating "development" is set

mithro/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware