Export BOARD_RADIO_IMAGE_PATH to Soong

The variable is used for radio partition generation in soong only build.

This change also removes the export of the no longer used variable
`RadioFilePath`.

Test: m nothing
Bug: 432817167
Bug: 434279647
Change-Id: Ife9faa2db6bd5aaec4bc1898fb0ff35bc8d0a6b4