rules.mk: add FILECMD variable

Some packages using Libtool have the possibility
of using this environment variable if set,
instead of the direct command for "file" using PATH,
or the need to patch a hard-coded path.

This is a new feature of Libtool 2.4.7

Ref: bf261073 ("tools/libtool: bump to 2.4.7")
Signed-off-by: Michael Pratt <mcpratt@pm.me>

rules.mk

@@ -252,6 +252,7 @@ TARGET_NM:=$(TARGET_CROSS)gcc-nm
 TARGET_CC:=$(TARGET_CROSS)gcc
 TARGET_CXX:=$(TARGET_CROSS)g++
 KPATCH:=$(SCRIPT_DIR)/patch-kernel.sh
+FILECMD:=$(STAGING_DIR_HOST)/bin/file
 SED:=$(STAGING_DIR_HOST)/bin/sed -i -e
 ESED:=$(STAGING_DIR_HOST)/bin/sed -E -i -e
 MKHASH:=$(STAGING_DIR_HOST)/bin/mkhash