oe-buildenv-internal: Add upper and lower case proxy vars to BB_ENV_EXTRAWHITE
Applications are inconsistent in their use of upper and lower case proxy
variables. Curl, for example, specifies NO_PROXY (not no_proxy) in the
man page (changed in 2009 [1]). Avoid proxy issues by ensuring both the
upper and lower case versions of each proxy variable are available in
the environment for the fetcher commands.
Add FTPS_PROXY and ftps_proxy to the list as well.
