kconfig: let 'shell' return enough output for deep path names

The 'shell' built-in only returns the first 256 bytes of the command's
output. In some cases, 'shell' is used to return a path; by bumping up
the buffer size to 4096 this lets us capture up to PATH_MAX.

The specific case where I ran into this was due to commit 1e860048c53e
("gcc-plugins: simplify GCC plugin-dev capability test"). After this
change, we now use `$(shell,$(CC) -print-file-name=plugin)` to return
a path; if the gcc path is particularly long, then the path ends up
truncated at the 256 byte mark, which makes the HAVE_GCC_PLUGINS
depends test always fail.

Signed-off-by: Brenda Streiff <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/kconfig/preprocess.c

···
 static char *do_shell(int argc, char *argv[])
 {
 	FILE *p;
-	char buf[256];
+	char buf[4096];
 	char *cmd;
 	size_t nread;
 	int i;