build system: drop PLATFORM_LINUX

PLATFORM_LINUX is a hidden configuration option which is disabled by
default and enabled at over a hundred locations for features that are
deemed to be Linux specific.

The only effect of PLATFORM_LINUX is to control compilation of
libbb/match_fstype.c.  This file is only needed by mount and umount.

Remove all references to PLATFORM_LINUX and compile match_fstype.c
if mount or umount is enabled.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
109 files changed