building: prerequisites: add e2tools and libusb-1.0-0-dev

ROCK Pi 4 needs e2tools to build the image and libusb-1.0-0-dev to
flash it.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

building/prerequisites.rst

@@ -26,6 +26,7 @@ available.
               cscope \
               curl \
               device-tree-compiler \
+              e2tools \
               expect \
               fastboot \
               flex \
@@ -44,6 +45,7 @@ available.
               libslirp-dev \
               libssl-dev \
               libtool \
+              libusb-1.0-0-dev \
               make \
               mtools \
               netcat \