lib/efi: Add new struct efi_mount

lib/efi: Add new struct efi_mount

To make it easier to manage EFI variables add a new struct efi_mount
that holds the path to the EFI file system mount and the EFI variable
name GUID. Update the lib/efi routines to use struct efi_mount.  Add
a new routine efi_check_mount based on the checks done in
platform-arm64.

This change to using struct efi_mount removes the static variable
efivarfs_path making the lib/efi routines stateless.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>