sandbox: Support executables for more phases

sandbox: Support executables for more phases

The SPL header has a function for obtaining the phase in capital letters,
e.g. 'SPL'. Add one for lower-case also, as used by sandbox.

Use this to generalise the sandbox logic for determining the filename of
the next sandbox executable. This can provide support for VPL.

Signed-off-by: Simon Glass <sjg@chromium.org>