fix build error in a shallow-cloned source tree

Cloning the repository with the --depth option is useful when you do
not need the full git history, but it fails to build core/version.c

  $ git clone --depth 1 git://git.ipxe.org/ipxe.git
  $ cd ipxe/src
  $ make
   [snip]
      [VERSION] bin/version.ipxe.dsk.o
  In file included from include/ipxe/features.h:6,
                   from core/version.c:33:
  <command-line>: error: unsigned conversion from ‘int’ to ‘unsigned char’ changes value from ‘9062’ to ‘102’ [-Werror=overflow]
  include/ipxe/dhcp.h:529:58: note: in definition of macro ‘DHCP_OPTION’
    529 | #define DHCP_OPTION( ... ) VA_ARG_COUNT ( __VA_ARGS__ ), __VA_ARGS__
        |                                                          ^~~~~~~~~~~
    ...

A shallow-cloned repository may not have any tag at all, then it returns
only a commit hash.

  $ git describe --tags --always --long --abbrev=1 --match "v*"
  9062

In such a case, VERSION_MINOR becomes empty, causing the build error.

If 'git describe' does not find any tag, let the version macros fall
back to the default:

  VERSION_MAJOR   = 1
  VERSION_MINOR   = 0
  VERSION_PATCH   = 0
  EXTRAVERSION    = +

Setting sha1 to GITVERSION is still useful.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

src/Makefile

@@ -208,8 +208,9 @@ install :
 #
 # Version number calculations
 #
-ifneq ($(wildcard ../.git),)
-VERSIONS := $(shell git describe --tags --always --long --abbrev=1 --match "v*")
+VERSIONS := $(if $(wildcard ../.git),$(shell git describe --tags --always --long --abbrev=1 --match "v*"))
+
+ifneq ($(filter v%, $(VERSIONS)),)
 VERSION_TUPLE	:= $(subst ., ,$(subst -, ,$(patsubst v%,%,$(VERSIONS))))
 VERSION_MAJOR	:= $(word 1,$(VERSION_TUPLE))
 VERSION_MINOR	:= $(word 2,$(VERSION_TUPLE))
@@ -225,6 +226,7 @@ VERSION_MAJOR	= 1
 VERSION_MINOR	= 0
 VERSION_PATCH	= 0
 EXTRAVERSION	= +
+GITVERSION	= $(VERSIONS)
 endif
 MM_VERSION	= $(VERSION_MAJOR).$(VERSION_MINOR)
 VERSION		= $(MM_VERSION).$(VERSION_PATCH)$(EXTRAVERSION)