fs: Create functions to load and allocate a file

This functionality current sits in bootstd, but it is more generally
useful. Add a function to load a file into memory, allocating it as
needed. Adjust bootstd to use this version.

Note: Tests are added in the subsequent patch which converts the 'cat'
command to use this function.

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