https://wiki.contextgarden.net/Symbols shows predefined symbol sets. The uni and wasysym are unknown. 

Hereby my example: 

\usesymbols[mvs]

\usesymbols[uni]

\usesymbols[wasysym]

%% \usesymbols[ifsym]

\starttext

\par Show entire set martinvogel 1

\showsymbolset[martinvogel 1]

\par Show YinYang symbol from martinvogel 2. \crlf

\symbol[martinvogel 2][YinYang]

%% \symbol[Alpine Symbol][Flag]

\stoptext

Console output

fonts > symbols > library 'mvs' loaded

fonts > symbols > library 'uni' is unknown

fonts > symbols > library 'wasysym' is unknown

The ultimate goal is to show a flag from the alpine symbol set from ifsym. However, I will settle with a flag from uni if it means I don't have to install files.

Kind regards,

Susanne