Makefile: Don't hide Gradle's progress

Currently, OSv's "make" builds both the kernel, using a normal Makefile,
and the sample management framework, using the "Gradle" tool.

We carefully hid Gradle's progress under one output line "GRADLE", but
this leads to the "make" process hanging for a long time - up to several
minutes, and all the puzzled user sees is the line "GRADLE". Users who
didn't go for coffee will likely kill the build at this point ;-) Even
worse, when Gradle fails, or just hangs (e.g., a bad or slow network
connection), the user won't even know why.

So let's just run Gradle normally, and let the user see its full output.
This output is rather pretty and organized, so no real reason to hide it.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>