docs/generic-loader: mention U-Boot and Intel HEX executable formats

docs/generic-loader: mention U-Boot and Intel HEX executable formats

The generic loader device supports the U-Boot and Intel HEX executable
formats in addition to the document raw and ELF formats.  Reword the
documentation to include these formats and explain how various options
depend on the executable format.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20180816145554.9814-1-stefanha@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>