tools: check liblzma in configure for rombios

We upgraded ipxe in 38ab99b2 ("ipxe: update to new commit"). That
version of ipxe requires liblzma to build.

Check that in configure and document this in README.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/configure.ac

@@ -173,6 +173,8 @@ AS_IF([test "x$enable_rombios" = "xyes"], [
     AX_PATH_PROG_OR_FAIL([AS86], [as86])
     AX_PATH_PROG_OR_FAIL([LD86], [ld86])
     AX_PATH_PROG_OR_FAIL([BCC], [bcc])
+    AC_CHECK_LIB([lzma], [lzma_version_number], [],
+        [AC_MSG_ERROR([Could not find lzma, needed to build rombios])])
     AC_DEFINE([HAVE_ROMBIOS], [1], [ROMBIOS enabled])
     rombios=y],[
     rombios=n