Drop make flags in soong_ui

MAKELEVEL and MAKEFLAGS were are both unset by makeparallel when invoked
from Make. In preparation for removing the Make wrapper (and
makeparallel), strip them in soong_ui as well.

Add MFLAGS for completeness.

Test: m -j
Change-Id: I029f9464d11b432e979b32ede360450f4facb038

ui/build/config.go

@@ -98,6 +98,11 @@ func NewConfig(ctx Context, args ...string) Config {
 		// Variables that have caused problems in the past
 		"DISPLAY",
 		"GREP_OPTIONS",
+
+		// Drop make flags
+		"MAKEFLAGS",
+		"MAKELEVEL",
+		"MFLAGS",
 	)
 
 	// Tell python not to spam the source tree with .pyc files.