tools/build: Fix root build target

tools/build: Fix root build target

The root Makefile 'build' rule recurses into the tools directory and invokes
the 'build' rule.  However, the tools Makefile doesn't contain a 'build' rule,
resulting in failure.

Introduce a 'build' rule identical to the existing 'all' rule.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>