buildman: make board selector argument a regex

A common use-case is to build all boards for a particular SoC. This can
be achieved by:

./tools/buildman/buildman -b mainline_dev tegra20

However, when the SoC is a member of a family of SoCs, and each SoC has
a different name, it would be even more useful to build all boards for
every SoC in that family. This currently isn't possible since buildman's
board selection command-line arguments are compared to board definitions
using pure string equality.

To enable this, compare using a regex match instead. This matches
MAKEALL's handling of command-line arguments. This enables:

(all Tegra)
./tools/buildman/buildman -b mainline_dev tegra

(all Tegra)
./tools/buildman/buildman -b mainline_dev '^tegra.*$'

(all Tegra20, Tegra30 boards, but not Tegra114)
./tools/buildman/buildman -b mainline_dev 'tegra[23]'

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>