crossgcc: `make clean` fails for SeaBIOS because IASL definition is not passed

[Paul Menzel]

Dear coreboot folks,

running `make clean` I get the following behavior (SeaBIOS 2062f2ba).

coreboot$ make clean
        The SeaBIOS project requires the 'iasl' package be installed.
        Many Linux distributions have this package.
        Try: sudo yum install iasl
        Or: sudo apt-get install iasl

Please install iasl and retry.
        Makefile:87: *** "Please upgrade the build environment".  Schluss.
        coreboot$ cd build/seabios/
        coreboot/build/seabios$ make clean
        The SeaBIOS project requires the 'iasl' package be installed.
        Many Linux distributions have this package.
        Try: sudo yum install iasl
        Or: sudo apt-get install iasl

Please install iasl and retry.
        Makefile:87: *** "Please upgrade the build environment".  Schluss.

$ more Makefile
        […]
        IASL:=iasl

# Default targets
        -include $(KCONFIG_CONFIG)

target-y = $(OUT) $(OUT)bios.bin
        target-$(CONFIG_BUILD_VGABIOS) += $(OUT)vgabios.bin

all: $(target-y)

# Make definitions
        .PHONY : all clean distclean FORCE

vpath %.c src vgasrc
        vpath %.S src vgasrc

################ Common build rules

# Verify the build environment works.
        TESTGCC:=$(shell CC="$(CC)" LD="$(LD)" IASL="$(IASL)" tools/test-build.sh)
        ifeq "$(TESTGCC)" "-1"
        $(error "Please upgrade the build environment")
        endif

So is there a way that the environment variables from coreboot are
passed to SeaBIOS? `make` with a SeaBIOS containing Marc’s commit
74f96123 »Add IASL definition to the Makefile.« works fine.

Thanks,

Paul