During e.g. cross-compiling, it is sometimes useful to be able to have
another binary than the default one as the compiler. This patch for the
build scripts lets "go" be the default value for the variable GO, and
the latter is then used as the go compiler.

Change-Id: I0adf7a661b26593d9b0ea902a61b631b80e76ae7
Signed-off-by: Joakim Roubert <joakimr@axis.com>