uefi: Implement path

UEFI paths can be of 4 types:
1. Absolute Shell Path: Uses shell mappings
2. Absolute Device Path: this is what we want
3: Relative root: path relative to the current root.
4: Relative

Absolute shell path can be identified with `:` and Absolute Device path
can be identified with `/`. Relative root path will start with `\`.

The algorithm is mostly taken from edk2 UEFI shell implementation and is
somewhat simple. Check for the path type in order.

For Absolute Shell path, use `EFI_SHELL->GetDevicePathFromMap` to
get a BorrowedDevicePath for the volume.

For Relative paths, we use the current working directory to construct
the new path.

BorrowedDevicePath abstraction is needed to interact with
`EFI_SHELL->GetDevicePathFromMap` which returns a Device Path Protocol
with the lifetime of UEFI shell.

Absolute Shell paths cannot exist if UEFI shell is missing.

Signed-off-by: Ayush Singh <ayush@beagleboard.org>

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