package/go: bump to version 1.12.1

package/go: bump to version 1.12.1

Go 1.12 was released on 2/25/2019: https://blog.golang.org/go1.12

Go 1.12.1 was released on 3/14/2019:
https://golang.org/doc/devel/release.html#go1.12.minor

Additional notes on how Go modules will evolve in 2019 are here:
https://blog.golang.org/modules2019

In Go 1.12, module support remains the same as in Go 1.11. GOPATH is scheduled
for deprecation in 1.13, however. The Buildroot Go package infrastructure should
therefore aim to migrate to a new, currently undefined, Go module friendly
compilation approach before Go 1.13 is released.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>