Add wrapper functions to convert strings to EFI_DEVICE_PATH

Add wrapper functions to convert strings to EFI_DEVICE_PATH

In anticipation of new functionality, create routines to convert char *
and a CHAR16 * to a EFI_DEVICE_PATH
	EFI_DEVICE_PATH *efi_name_to_devpath(const char *path);
	EFI_DEVICE_PATH *efi_name_to_devpath16(CHAR16 *path);
	void efi_devpath_free(EFI_DEVICE_PATH *dp);
The first two return an EFI_DEVICE_PATH for the passed in paths. The
third frees up the storage the first two return when the caller is
done with it.

Differential Revision: https://reviews.freebsd.org/D19971