package/dropbear: add option to provide file with localoptions.h

As discussed in https://patchwork.ozlabs.org/patch/1104071/, this
commit adds a new option that allows the user to provide a file that
contains custom definitions to tweak the Dropbear configuration. It
will be appended to Dropbear's localoptions.h file before the build.

The patch was tested successfully with the DO_MOTD option.

Suggested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
[Thomas: tweak commit log, rename config option.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/dropbear/Config.in

@@ -66,4 +66,11 @@ config BR2_PACKAGE_DROPBEAR_LEGACY_CRYPTO
 	    DSA public keys
 	    Diffie-Hellman Group1 key exchange
 
+config BR2_PACKAGE_DROPBEAR_LOCALOPTIONS_FILE
+	string "path to custom localoptions.h definitions file"
+	help
+	  Path to a file whose contents will be appended to Dropbear
+	  localoptions.h. It can be used to tweak the Dropbear
+	  configuration.
+
 endif

package/dropbear/dropbear.mk

@@ -124,6 +124,14 @@ ifneq ($(BR2_PACKAGE_DROPBEAR_LASTLOG),y)
 DROPBEAR_CONF_OPTS += --disable-lastlog
 endif
 
+DROPBEAR_LOCALOPTIONS_FILE = $(call qstrip,$(BR2_PACKAGE_DROPBEAR_LOCALOPTIONS_FILE))
+ifneq ($(DROPBEAR_LOCALOPTIONS_FILE),)
+define DROPBEAR_APPEND_LOCALOPTIONS_FILE
+	cat $(DROPBEAR_LOCALOPTIONS_FILE) >> $(@D)/localoptions.h
+endef
+DROPBEAR_POST_EXTRACT_HOOKS += DROPBEAR_APPEND_LOCALOPTIONS_FILE
+endif
+
 define DROPBEAR_INSTALL_TARGET_CMDS
 	$(INSTALL) -m 755 $(@D)/dropbearmulti $(TARGET_DIR)/usr/sbin/dropbear
 	for f in $(DROPBEAR_TARGET_BINS); do \