meta-google: gbmc-update: Migrate to curl

We want to remove wget from busybox so we need an alternate fetcher.

Tested: Ran locally against an installer URL to verify the behavior

Change-Id: Ib3a00002d7d2d02bd6b29e24f0dbe2c7c9243514
Signed-off-by: William A. Kennington III <wak@google.com>