purgatory: Use standalond CFLAGS

There has been a lot of workarounds for purgatory disabling many
specified CFLAGS that will break purgatory. It will be better to not
let the CFLAGS used to compile purgatory honor the CFLAGS from
environment variables. So we will have stable CFLAGS for purgatory.

If anyone still wants to change purgatory CFLAGS, PURGATORY_EXTRA_CFLAGS
is still honored.

Signed-off-by: Simon Horman <horms@verge.net.au>