Fix variable name.
Add a way to specify environment variables on the command-line.

If we do:

    mksb FOO1=BAR1 FOO2=BAR2 FOO3=BAR3

then the following will be appended to the generated .sandbox-env file:

FOO1=BAR1
FOO2=BAR2
FOO3=BAR3

In addition, "make checkout" will be invoked as:

make checkout FOO1=BAR FOO2=BAR2 FOO3=BAR3
Add an --initial-branch flag, for specifying the checkout branch for
the initial repository only.

The -b flag specifies the checkout branch for all repositories
   (make checkout BRANCH=somebranch)

The -t flag specifies the checkout tag for all repositories
   (make checkout TAG=sometag)

Since the FreeNAS repositories do not use the same
branch names across all repositories (yet), we need a way to
specify the branch for at least the initial repository only.
Remove outdated comments about the repo cache.  We will re-implement
the repo cache later.