gsctool: verbose flag should print debug messages
gsctool: verbose flag should print debug messages

Currently the verbose flag is stored in `verbose_mode` but
never used.  Print conditionally on this flag within a new
`debug` function.

BUG=None
TEST=try running various commands with --verbose
     check for verbose output
BRANCH=master

Change-Id: Iadaf1a91ea1b02b0638e6364077c2c148f26e03c
Reviewed-on: https://chromium-review.googlesource.com/1351926
Commit-Ready: Joel Kitching <c2feea8c1334a0d339e816ae4c7c9ecfa9b30a91@chromium.org>
Tested-by: Joel Kitching <c2feea8c1334a0d339e816ae4c7c9ecfa9b30a91@chromium.org>
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec