packages: standardize values of several control variables for build.sh

Variables

 TERMUX_PKG_PLATFORM_INDEPENDENT
 TERMUX_DEBUG
 TERMUX_PKG_HAS_DEBUG
 TERMUX_PKG_ESSENTIAL
 TERMUX_SUBPKG_ESSENTIAL
 TERMUX_PKG_NO_STATICSPLIT
 TERMUX_PKG_BUILD_IN_SRC
 TERMUX_PKG_FORCE_CMAKE
 TERMUX_PKG_HOSTBUILD

should not accept arbitrary values for marking them "enabled". Instead
they should accept boolean values which makes them easier to handle and
also makes their meaning clear.

build-package.sh should make decision based on variable's value but not on
whether it is set or empty.

%ci:no-build

packages/vim-python/build.sh

@@ -23,7 +23,7 @@ vim_cv_tty_group=world
 --without-x
 --with-tlib=ncursesw
 "
-TERMUX_PKG_BUILD_IN_SRC="yes"
+TERMUX_PKG_BUILD_IN_SRC=true
 TERMUX_PKG_RM_AFTER_INSTALL="
 bin/rview
 bin/rvim
@@ -54,7 +54,7 @@ termux_step_pre_configure() {
 termux_step_pre_configure() {
 	# Certain packages are not safe to build on device because their
 	# build.sh script deletes specific files in $TERMUX_PREFIX.
-	if [ -n "$TERMUX_ON_DEVICE_BUILD" ]; then
+	if $TERMUX_ON_DEVICE_BUILD; then
 		termux_error_exit "Package '$TERMUX_PKG_NAME' is not safe for on-device builds."
 	fi