Makefile: don't export O to the environment

Several projects use the kernel style O=<dir> syntax to build out of tree,
and atleast uClibc doesn't check that it was explictly passed on the command
line, so setting it in the environment breaks the build.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -291,7 +291,7 @@ export TARGET_DIR
 export STAGING_DIR
 export HOST_DIR
 export BINARIES_DIR
-export BASE_DIR O   # O and BASE_DIR are the same
+export BASE_DIR
 
 #############################################################
 #

docs/manual/customize-rootfs.txt

@@ -43,7 +43,7 @@ there are a few ways to customize the resulting target filesystem.
       xref:generic-package-reference[]
     - +BINARIES_DIR+: the place where all binary files (aka images) are
       stored
-    - +BASE_DIR+ (or +O+): the base output directory
+    - +BASE_DIR+: the base output directory
 
 * A special package, 'customize', stored in +package/customize+ can be
   used. You can put all the files that you want to see in the final