sandbox: capsule: binman: generate some capsules as part of build

sandbox: capsule: binman: generate some capsules as part of build

Currently, all the capsules for the sandbox platform are generated at
the time of running the capsule tests. To showcase generation of
capsules through binman, generate all raw(non FIT payload) capsules
needed for the sandbox platform as part of the build. This acts as an
illustrative example for generating capsules as part of a platform's
build.

Make corresponding change in the capsule test's configuration to get
these capsules from the build directory.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>