mesh: shell: Add support for decoding hex strings
mesh: shell: Add support for decoding hex strings

Add helper to decode arbitrary-length hex strings. The first user is
the AppKeyAdd handler.

X-Original-Commit: 2950084e1d0e8d99b6a6e608ca58603aedbdede8

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble