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>

coreutils/date.c

@@ -37,7 +37,6 @@
 //config:	bool "Support %[num]N nanosecond format specifier"
 //config:	default n # stat's nanosecond field is a bit non-portable
 //config:	depends on DATE
-//config:	select PLATFORM_LINUX
 //config:	help
 //config:	Support %[num]N format specifier. Adds ~250 bytes of code.
 //config:

coreutils/stat.c

@@ -31,7 +31,6 @@
 //config:	bool "Enable display of filesystem status (-f)"
 //config:	default y
 //config:	depends on STAT
-//config:	select PLATFORM_LINUX # statfs()
 //config:	help
 //config:	Without this, stat will not support the '-f' option to display
 //config:	information about filesystem status.