run: introduce have_board helper function

The new helper function returns a boolean value analoque to 'have_spec' if the
BOARD variable corresponds to the given value. It shall replace [have_spec <board>]
declarations in run-scripts.

Ref #3971

tool/run/boot_dir/linux

@@ -31,7 +31,7 @@ proc run_boot_dir {binaries} {
 	# Save config part of the image.elf for easy inspection
 	exec cp -f [run_dir]/genode/config [run_dir].config
 
-        if {[board] == "pc"} {
+        if {[have_board pc]} {
 
             build_initrd $binaries
 

tool/run/qemu.inc

@@ -44,9 +44,7 @@ proc append_qemu_nic_args { { extra_netdev_args "" } } {
 # XXX should by removed in favor of [have_include "exec/qemu"]
 #
 proc is_qemu_available { } {
-	if {[expr [have_spec linux] && {"[board]"} == {"linux"}]} { return false }
-
-	if {[have_spec rpi]} {
+	if {[have_board rpi] || [have_board linux]} {
 		puts stderr "skipping execution because platform is not supported by qemu"
 		return false
 	}

tool/run/run

@@ -835,6 +835,10 @@ proc board { } {
 	return $board_var
 }
 
+##
+# Return whether the board equals to the given string
+#
+proc have_board {board} { global board_var; return [expr {$board_var} eq {$board}] }
 
 ##
 # Determine GDB executable installed at the host