Make $TERMUX_DEBDIR configurable

Allows building debs outside of the Termux source directory

build-package.sh

@@ -262,6 +262,7 @@ termux_step_setup_variables() {
 	: "${TERMUX_ANDROID_HOME:="/data/data/com.termux/files/home"}"
 	: "${TERMUX_DEBUG:=""}"
 	: "${TERMUX_PKG_API_LEVEL:="21"}"
+	: "${TERMUX_DEBDIR:="${TERMUX_SCRIPTDIR}/debs"}"
 
 	if [ "x86_64" = "$TERMUX_ARCH" ] || [ "aarch64" = "$TERMUX_ARCH" ]; then
 		TERMUX_ARCH_BITS=64
@@ -288,7 +289,6 @@ termux_step_setup_variables() {
 	TERMUX_D8=$ANDROID_HOME/build-tools/$TERMUX_ANDROID_BUILD_TOOLS_VERSION/d8
 
 	TERMUX_COMMON_CACHEDIR="$TERMUX_TOPDIR/_cache"
-	TERMUX_DEBDIR="$TERMUX_SCRIPTDIR/debs"
 	TERMUX_ELF_CLEANER=$TERMUX_COMMON_CACHEDIR/termux-elf-cleaner
 
 	export prefix=${TERMUX_PREFIX}