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>
c1790b14bc