posix file: Add an `Offset` parameter and make `Len` optional

Give Map() an `Offset` parameter that will be passed to mmap(). Make the
`Len` parameter of Map() optional. If it's left at the default, `0`, map
until the end of the file (up to SIZE_MAX bytes).

Change-Id: I7b062ce1e9cddab87a66a2aedc8b64e4e8524d9f
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/20553
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
3 files changed