autotools.bbclass: fix issue with native and sed
autotools.bbclass: fix issue with native and sed

STAGING_DIR_HOST is "" when using native bbclass:
native.bbclass:STAGING_DIR_HOST = ""
But in autotools.bbclass there is:
 sed -i -e s:${STAGING_DIR_HOST}::g $i
which result in the following code in run.autotools_prepackage_lamangler
 sed -i -e s:::g $i
which makes libxml2-native fail like this:
 | sed: -e expression #1, char 0: no previous regular expression

I discussed it on IRC:
Feb 11 17:41:29 <RP>    GNUtoo: We should only be doing that is STAGING_DIR_HOST isn't empty I guess

So I made a temporary fix which only workarround,because some issues persist:
Feb 11 17:41:02 <pb_>   RP: heh, I wonder whether paths with colons in would be considered valid :-}

Signed-off-by: Denis 'Gnutoo' Carikli <dc53f704238ac18d4170dcdce16b661f43d75df3@no-log.org>

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